# Calabi-Yau manifold

 Shing-Tung Yau (2009), Scholarpedia, 4(8):6524. doi:10.4249/scholarpedia.6524 revision #170091 [link to/cite this article]
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Curator: Shing-Tung Yau

Calabi-Yau manifolds are compact, complex Kähler manifolds that have trivial first Chern classes (over $$\mathbb{R}$$). In most cases, we assume that they have finite fundamental groups. By the conjecture of Calabi (1957) proved by Yau (1977; 1979), there exists on every Calabi-Yau manifold a Kähler metric with vanishing Ricci curvature.

Currently, research on Calabi-Yau manifolds is a central focus in both mathematics and mathematical physics. It is partially propelled by the prominent role the Calabi-Yau threefolds play in superstring theories. While many beautiful properties of Calabi-Yau manifolds have been discovered, more questions have been raised and probed. The landscape of various constructions, theories, conjectures, and above all the fast pace of progress in this subject, have made the research of Calabi-Yau manifolds an extremely active research field both in mathematics and in mathematical physics.

## General Constructions of Complete Ricci-Flat Metrics in Kähler Geometry

### The Ricci tensor of Calabi-Yau manifolds

A complex manifold is a topological space covered by complex coordinate charts such that the transition between overlapping charts are holomorphic; a Hermitian metric on a complex manifold is a smooth assignment of Hermitian inner product structures on the holomorphic tangent spaces of the manifold; a Hermitian metric is called a Kähler metric if near every point the Hermitian metric is approximated by a flat metric up to second order. In a holomorphic coordinate chart with coordinate variables $$(z_1,\cdots,z_n)\ ,$$ a Hermitian metric has its associated Hermitian form $\omega=\frac{\sqrt{-1}}{2}\sum_{}g_{i\bar{j}}dz^{i}\wedge d\bar{z}^{j}.$ The second order approximation property is that for each holomorphic co-ordinates so that the derivatives of the functions $$g_{i\bar{j}}$$ vanish at that point. It can be shown that this is equivalent to saying that the Hermitian form is closed. In such a case, we call the Hermitian form the Kähler form of the Kähler metric.

Given any Kähler metric, one defines its full curvature tensor by certain expressions of covariant derivatives of the metric; the Ricci curvature is a partial contraction of the full curvature tensor. This gives a tensor which is of the same type as the Kähler form. In local coordinates, $Ric=\sqrt{-1}\sum R_{i\bar j}dz^i\wedge d\bar z^j.$ We call a Kähler metric Ricci-flat if its Ricci tensor vanishes identically.

### The Calabi conjecture

According to a well-known theorem of Chern, the Ricci form divided by $$2\pi$$ is a $$(1,1)$$-form that represents the first Chern class of a compact complex manifold. Rooted in his attempt to find canonical Kähler metrics for a Kähler manifold, in 1954, E. Calabi (Calabi, 1957) proposed his celebrated conjecture.

Conjecture. To every closed $$(1,1)$$-form $$\frac{1}{2\pi}C_1(X)$$ representing the first Chern class $$c_1(X)$$ of a Kähler manifold $$X\ ,$$ there is a unique Kähler metric in the same Kähler class whose Ricci tensor (form) is the closed $$(1,1)$$-form $$C_1(X)\ .$$

In case the complex manifold has vanishing first Chern class, the zero form represents the first Chern class of the manifold. The Calabi conjecture implies the existence of a unique Ricci-flat Kähler metric in every Kähler class.

Early on, Calabi realized that his conjecture can be reduced to a complex Monge-Ampère equation.

### Yau's theorem

By the late 1960s, many were doubtful of the Calabi conjecture. Some attempted to use a reduction theorem of Cheeger and Gromoll (1971) to construct counterexamples to the Conjecture. Using the reduction theorem and assuming the conjecture, Yau announced the following splitting theorem in his 1973 lecture at the Stanford geometry conference: Every compact Kähler manifold with vanishing Ricci curvature can be covered by a metric product of a torus and a simply connected manifold with a Ricci-flat Kähler metric. He then used this theorem to produce a "counterexample" to the conjecture. The "counterexample" was soon discovered to be flawed; Yau withdrew his Stanford lecture. (The flaw was due to the mistaken assumption that manifolds with numerically non-negative anti-canonical divisor admits a first Chern form which is pointwise non-negative.)

In 1976, Yau (Yau, 1977; Yau, 1979) proved the Calabi conjecture by solving the complex Monge-Ampère equation for a real valued function $$\phi$$ $\det\left( g_{i\bar{j}}+\frac{\partial^{2}\phi}{\partial z^{i}\partial\bar {z}^{j}}\right) =e^{f}\det\left( g_{i\bar{j}}\right),$ where $$e^f$$ is any smooth function of average 1 and $$g_{i \bar j} + \partial_i {\partial}_{\bar j}\phi$$ is required to be positive definite. The solution $$\phi$$ of the above equation ensures that the new Kähler metric $\omega+\sqrt{-1}\partial\bar\partial\phi$ can attain any Ricci (curvature) form in the class referred to in the Calabi conjecture.

### Calabi-Yau manifolds and Calabi-Yau metrics

The first application to Yau's proof of Calabi conjecture is the existence of Ricci-flat Kähler metric on every compact complex Kähler manifold with trivial canonical class. (Trivial canonical class is equivalent to the existence of a nowhere vanishing holomorphic volume form, which is equivalent to that the top wedge power of the holomorphic cotangent bundle is the trivial line bundle.) The converse is also true: any Ricci-flat simply connected Kähler manifold has trivial canonical line class. This proves the existence and provides a criterion for Kähler Calabi-Yau manifolds.

By convention, Calabi-Yau manifolds exclude those with infinite fundamental groups. The Ricci-flat metrics on Calabi-Yau manifolds are called Calabi-Yau metrics.

The existence of Calabi-Yau metrics has other important consequences. In his paper (Yau, 1977; Yau, 1979), Yau demonstrated that for a Calabi-Yau manifolds $$(X,\omega)\ ,$$ using the Chern-Weil form representing the second Chern form $$C_{2}(X)$$ in terms of the curvature tensor $$\mathrm{Rm}$$ of a Calabi-Yau Kähler metric of $$X\ ,$$ one gets $\int_{X}C_{2}(X)\wedge\omega^{n-2}=C\int _{X}\left\vert \mathrm{Rm}\right\vert ^{2}\text{vol}\geq0$ for some positive constant $$C\ .$$ Thus the Chern number $$c_{2}(X)\cap[\omega]^{n-2}$$ is non-negative. Moreover, when it is zero, we have $$\mathrm{Rm}=0\ ,$$ and therefore $$X$$ is covered by the Euclidean space $$\mathbb{C}^{n}\ .$$

Another application is the reduction of holonomy groups of Calabi-Yau manifolds. One important consequence of a Calabi-Yau metric is that the parallel transports along contractible closed loops preserve the metric and the holomorphic volume form. This implies that the restricted holonomy group of a Calabi-Yau manifold is a subgroup of $$SU(n)\ ,$$ the group of special unitary transformations.

Sometimes, this group can be strictly smaller than $$SU(n)\ .$$ Following the Bochner technique on Calabi-Yau manifolds, every holomorphic $$(p,0)$$-form is parallel. Such a form then reduces the holonomy group from $$SU(n)$$ to a smaller subgroup. Thus if the holonomy group of $$X$$ is the full $$SU(n)\ ,$$ then the Dolbeault cohomology group $$H^{p,0}(X)=0$$ for $$1\leq p\leq n-1$$ and $$H^{n,0} (X)$$ is one dimensional, spanned by the holomorphic volume form $$\Omega$$ of $$X\ .$$

The statement of the structure theorem was known to many people, including the announcement made by Yau in 1973. It was also announced by Kobayashi, by Michelsohn (1982), and appeared in a subsequent survey paper of Beauville (1999). It states that any compact, complex Kähler manifold with trivial canonical class has a cover that is a metric product of a complex Euclidean space with copies of manifolds with holonomy groups $$SU(m)$$ and copies of manifolds with holonomy groups $$Sp(m/2)\ ;$$ here $$m$$'s are the dimensions of the corresponding manifolds.

The proof is based on the above-mentioned splitting theorem of Cheeger-Gromoll and also an argument of Calabi who drew some consequences on the first Betti number.

### Examples of compact Calabi-Yau manifolds

By Yau's solution to the Calabi conjecture, finding (non-hyperkähler) Calabi-Yau manifolds is equivalent to finding smooth projective varieties of trivial canonical class. The easiest way to do this is, in complex dimension $$n\ ,$$ is to take a smooth hypersurface of degree $$n+2$$ in $$\mathbb{P}^{n+1}\ .$$ Thus the first example of a Calabi-Yau threefold is the smooth quintic in the complex projective space $$\mathbb{P}^4\ .$$ Due to a condition imposed by superstrings theories, Calabi-Yau threefolds having Euler characteristic $$\chi=\pm6$$ and non-trivial fundamental group play a special role. Such examples were first discovered and announced by Yau in a lecture given at the 1985 Argonne conference (Yau, 1985) as the $$\mathbb{Z}_3$$ quotient of an intersection of two cubics and a hypersurface of bi-degree $$(1,1)$$ in the product $$\mathbb{P}^3\times\mathbb{P}^3\ .$$ More examples were found later by Tian and Yau; due to an observation of Greene and Kirklin (1987), these examples are deformation-equivalent to the one found by Yau. A systematic search for Calabi-Yau threefolds with $$\chi=\pm6$$ turned up no essentially new example among complete intersections in toric varieties (Candelas, Lutken and Schimmrigk, 1988).

After Yau's examples of complete intersection Calabi-Yau threefolds in product of projective spaces, various groups, notably the group in University of Texas, employed computer algorithm to search for new examples (Hübsch, 1987; Green and Hübsch, 1987; Candelas, Dale, Lutken and Schimmrigk, 1988). Soon after, about 8000 constructions, with 256 distinct Hodge diamonds were found (Green, Hübsch and Lütken, 1989). This pool grew by about 3 orders of magnitude by embedding in products of weighted  complex projective spaces (Candelas, Lynker and Schimmrigk, 1990). Such threefolds typically have finite quotient singularities inherited from the weighted projective spaces, and a minimal blow-up following Roan and Yau's 1987 construction  (Roan and Yau, 1987) is understood to provide smooth models. Roan and Yau also proposed to use toric method to construct more examples. This was carried out by Batyrev and Borisov for complete intersections in toric varieties (Batyrev and Borisov, 1996).

Yau conjectured that there are finitely many topological types of Calabi-Yau manifolds in each dimension. This conjecture is still open. By a rough count, by 2002, over 473 million toric embeddings of Calabi-Yau threefolds were constructed, with over 30,000 distinct Hodge diamonds (Kreuzer and Skarke, 2002a; Kreuzer and Skarke, 2002b). Based on Wall's theorem  (1966) one sees that complete intersections of hypersurfaces in products of projective spaces produce at least 2590 distinct diffeomorphism classes (Candelas and He, 1990). It is worth noticing that so far all Calabi-Yau threefolds can be constructed as deformations or small resolutions of complete intersections of toric varieties. The non-trivial check of this for Calabi-Yau complete intersections in Grassmannians and flag varieties was done by Batyrev, Ciocan-Fontanine, Kim and van Straten (1998; 2000).

### Noncompact Calabi-Yau manifolds

Immediately after his proof of the Calabi conjecture, Yau generalized the construction of Calabi-Yau manifolds to non-compact Kähler manifolds. He presented this result in his plenary lecture in the 1978 Helsinki International Congress of Mathematics.

The construction is that in the complement $$M\setminus S$$ of an effective anticanonical divisor $$S$$ in a compact Kähler manifold $$M\ ,$$ suppose that the first Chern class of $$M$$ is either positive or trivial in a neighborhood of $$S\ :$$ then there is a complete Ricci-flat metric on $$M\setminus S$$ if $$S$$ is connected and geometrically stable.

In case $$S$$ is nonsingular and connected and assuming $$S$$ admits a Kähler Einstein metric with either positive or zero scalar curvature, the detail of the generalization was presented in a joint paper of Tian and Yau (1990; 1991). Around the same time, Bando and Kobayashi (1988; 1990) worked out some more restrictive cases. The condition that $$S$$ should be geometrically stable was added on after the Helsinki Congress. When $$S$$ is singular, the definition need to be clarified.

If the complement $$M\setminus S$$ admits a complete Ricci-flat metric, $$S$$ has to be connected unless the complement is a product of the complex line with other manifolds. Based on his result on the volume growth of complete manifolds with non-negative Ricci curvature and compactification of complete Kähler manifolds, Yau conjectured that his construction gives all examples of noncompact complete Kähler Ricci-flat metrics with connected end. There are counterexample to this conjecture: one such example was pointed out by Anderson, Kronheimer and LeBrun (1989). The conjecture remains open assuming the manifolds are of finite topological type.

Explicit Calabi-Yau metrics have been constructed in many cases when symmetries are present. First there is the explicit Eguchi-Hanson metric (Eguchi and Hanson, 1978) on the cotangent bundle $$T^{\ast}S^{2}$$ of the two sphere. Viewing this as the canonical line bundle over $$\mathbb{P}^1\ ,$$ Calabi (1979) constructed a complete Calabi-Yau metric on the total space $$X=K_{B}$$ of (a fraction of) the canonical line bundle of a positive Kähler-Einstein manifold $$B\ .$$ Later, Futaki (2007) generalized to that $$B$$ is any toric Fano manifold.

Candelas and de la Ossa (1990) constructed a one-parameter family of explicit Calabi-Yau metrics on affine quadrics $$X_{t}$$ in $$\mathbb{C}^{4}$$ (for nonzero $$t$$) given by the equation $$z_{0}^{2}+z_{1}^{2}+z_{2}^{2} +z_{3}^{2}=t\ ,$$ by reducing the Ricci-flat equation to an ODE. These manifolds are all diffeomorphic to the cotangent bundle $$T^{\ast}S^{3}$$ of the three sphere. The parameter  of this family corresponds to the size of $$S^{3}\ .$$ This was generalized to all dimensions by Stenzel (1993).

Candelas and de la Ossa (1990) also constructed a one-parameter family of explicit Calabi-Yau metrics on the total space of the vector bundle $$X=\mathcal{O}(-1)\oplus\mathcal{O}(-1)$$ over $$\mathbb{P}^1\ .$$ As the size of $$\mathbb{P}^1$$ shrinks to zero, the total space $$X$$ degenerates to a cone threefold $$X_{0}$$ with rational double point singularity $X_{0}=\left\{ z_{0}^{2}+z_{1}^{2}+z_{2}^{2}+z_{3}^{2}=0\right\} \subset\mathbb{C}^{4}\text{.}$ This singular space has two different small resolutions $$X_+$$ and $$X_-\ ,$$ both have total space $$\mathcal{O}(-1)\oplus\mathcal{O}(-1)\ .$$ The pair $$(X_+,X_-)$$ is a local model of a simple flop, a birational change of threefolds.

If one varies the quadratic equation by adding a small nonzero $$t\ ,$$ then it defines the smooth hypersurface $$X_{t}\ .$$ The change from $$X_{\pm}$$ to $$X_{t}$$ is called an extremal transition; it is the basic building block for topological changes of Calabi-Yau threefolds.

### Calabi-Yau cones: Sasaki-Einstein manifolds

An important class of non-compact and possibly incomplete Calabi-Yau manifolds are Calabi-Yau cones. The metric cone over a compact Riemannian manifold $$(S,g)$$ is defined to be $$(C(S)=\mathbb{R}_{+}\times S, \bar{g}=dr^{2} + r^{2} g)\ ,$$ where $$r>0$$ is a coordinate on $$\mathbb{R}_{+}\ .$$ If the dimension of this cone is $$2n$$ and the (restricted) holonomy group of $$(C(S),\bar{g})$$ is contained in $$SU(n)\ ,$$ then the manifold $$(S,g)$$ is called Sasaki-Einstein. In particular, since the cone is Ricci-flat it follows that $$(S,g)$$ is a $$(2n-1)$$-dimensional Einstein manifold of positive Ricci curvature, $$\mathrm{Ric}_{g} = 2(n-1)g\ .$$ In fact, many of the complete non-compact Calabi-Yau manifolds referred to above are asymptotic to such a cone, meaning that they are modelled at infinity by the large $$r$$ (complete) end of the cone. Sasaki-Einstein manifolds in low dimensions are also important in string theory, and in particular in the AdS/CFT correspondence. For example, the latter conjectures that to every Sasaki-Einstein 5-manifold there is an associated superconformal field theory on $$\mathbb{R}^{4}\ .$$ Much work has gone into understanding this correspondence, and the relationship between Sasaki-Einstein geometry and superconformal field theory.

The definition above is easily generalized: if the metric cone has (restricted) holonomy contained in $$U(n)\ ,$$ so that the cone is Kähler, the manifold $$(S,g)$$ is said to be a Sasakian manifold (Sasaki, 1960). These manifolds should be viewed as odd-dimensional analogs of Kähler manifolds. A Sasakian manifold inherits a strictly pseudo-convex hypersurface-type CR structure from the complex structure of the cone. Sasakian manifolds are also equipped with a unit norm Killing vector field $$\xi\ ,$$ called the Reeb vector field, defined as the restriction of $$J(r\partial/\partial r)$$ to $$\{r=1\}\cong S\subset C(S)\ ,$$ where $$J$$ is the complex structure tensor of the Kähler cone. The dual one-form $$\eta(X)=g(\xi,X)$$ is a contact form on $$S\ .$$ The flow of $$\xi$$ defines a one-dimensional foliation of $$S\ ,$$ and it turns out that the transverse leaf space is Kähler. Indeed, $$(S,g)$$ is Sasaki-Einstein if and only if this transverse Kähler structure, with transverse metric $$g^{T}\ ,$$ is Kähler-Einstein with positive Ricci curvature, $$\mathrm{Ric}_{g^{T}}=2ng^{T}\ .$$ Sasakian manifolds may be classified according to the global properties of this foliation. If the orbits of $$\xi$$ all close, thus defining a locally free circle action on $$S\ ,$$ the Sasakian manifold is said to be quasi-regular, and the leaf space is naturally a Kähler orbifold. In the special case that the circle action is free, the Sasakian manifold is said to be regular, and the leaf space is a Kähler manifold. If there is a non-closed orbit of $$\xi$$ the Sasakian manifold is said to be irregular.

The simplest example of a Sasaki-Einstein manifold is the round sphere, viewed as the unit sphere in $$\mathbb{C}^{n}$$ equipped with its flat Kähler metric. This is regular, with the Kähler-Einstein leaf space being $$\mathbb{CP}^{n-1}$$ equipped with its Fubini-Study metric. The study of regular Sasaki-Einstein manifolds is in fact essentially equivalent to the study of positive (Fano) Kähler-Einstein manifolds. Boyer and Galicki (2005; 2008) and their collaborators have constructed large classes of quasi-regular Sasaki-Einstein manifolds by constructing appropriate Kähler-Einstein orbifold leaf spaces. These are typically realized as weighted projective varieties, and the continuity method is used to prove existence, see (Wang and Zhu, 2004; Donaldson, 2008) for the existence of Kähler-Einstein metrics on toric varieties. Boyer, Galicki and Kollár (2005) have also shown the existence of numerous Sasaki-Einstein metrics on standard and exotic spheres using the Kähler-Einstein metrics on certain types of Fano orbifolds constructed by Demailly and Kollár (2001).

The first examples of irregular Sasaki-Einstein manifolds were constructed by Gauntlett, Martelli, Sparks and Waldram (2004); these authors constructed infinitely many explicit quasi-regular and irregular Sasaki-Einstein metrics on $$S^{2}\times S^{3}\ .$$ Recently, Futaki, Ono and Wang (2006) have proven the existence of toric Sasaki-Einstein metrics, following earlier work of Martelli, Sparks and Yau (2008). In this case the Kähler cone is the smooth part of an affine toric variety. Finally, Gauntlett, Martelli, Sparks and Yau (2007) have described some simple obstructions to the existence of Sasaki-Einstein metrics, which also give new obstructions to the existence of Kähler-Einstein metrics on Fano orbifolds.

### The balanced condition on Calabi-Yau metrics

After observing that the tangent bundles of Calabi-Yau manifolds are stable with respect to any polarization, Yau conjectured that the Calabi-Yau manifolds are also stable in the sense of geometric invariant theory. This was first openly discussed in the problem session in the UCLA geometry conference in 1990, where he proposed to approximate the Ricci-flat metric of a Calabi-Yau manifold $$X$$ by the induced metric from embedding $$X$$ into complex projective space by powers of an ample line bundle, and suggested that the action of the projective linear group on the embedding would link the stability of the manifold with the existence of the Ricci-flat metric.

Yau (1986) initiated the program of approximating Calabi-Yau metrics (and more generally KE-metrics) by embeddings. Under his guidance, Tian (Tian, 1990) wrote his thesis on the $$C^2$$ convergence of the pullback Fubini-Study metric via projective embeddings. (The $$C^\infty$$ convergence was proved in Ruan's Harvard thesis (Ruan, 1998). See also Dai, Liu and Ma (2006).) The refined structure of this embedding was investigated by Catlin (1999), Zelditch (1998) and Lu (2000). For the problem of finding a canonical position in embedding a Calabi-Yau manifold in projective space, the balanced condition introduced by Bourguignon, Li and Yau (1994) played a crucial role. (The idea of Bourguignon-Li-Yau was based on the concept of the conformal area introduced by Li and Yau (1982).) Following their idea, Luo (Luo, 1998) in his MIT thesis (under the guidance of Yau) generalized the notion of balanced embeddings to all projective manifolds and related the concept to questions of geometric stability. The relation between geometric stability and the balanced condition was also studied by Zhang (Zhang, 1996).

This program of Yau was carried out by Donaldson (2001; 2005) in more precise manner. Donaldson showed that for the sequence of balanced embeddings of a Calabi-Yau manifold into projective spaces via increasing powers of an ample line bundle, the sequence of normalized induced metrics converges to the Ricci-flat metric of $$X$$ (Donaldson, 2001). (See Liu and Ma, ( 2007) for some clarifications.) Based on the balanced embeddings, he then developed an algorithm to numerically approximate the Ricci-flat metrics of K3 surfaces. (Donaldson ( 2005) ; See Liu and Ma, ( 2007) for some clarifications.) This algorithm was generalized by Douglas, Karp, Lukic and Reinbacher, (2008), and subsequently by Braun, Brelidze, Douglas and Ovrut, (2008a; 2008b) to approximate the Ricci-flat metrics on various projective Calabi-Yau threefolds.

The work of Donaldson also showed that every Calabi-Yau manifold is asymptotically Chow stable, proving partially Yau's conjecture on the stability of Kähler Einstein manifolds (Donaldson, 2001).

## Moduli and Arithmetic of Calabi-Yau Manifolds

### Moduli of K3 surfaces

Two dimensional Calabi-Yau manifolds are called K3 surfaces. Moduli of K3 surfaces are classically known to be smooth. It has a modular description based on the Hodge structures on the K3 surfaces.

On any K3 surface $$X\ ,$$ the middle cohomology group $$H^{2}( X,\mathbb{Z})$$ is a free Abelian group of rank $$22$$ and coupled with the (intersection) quadratic form $$\left\langle \cdot,\cdot\right\rangle$$ the lattice $$(H^2(X,\mathbb{Z}) \left\langle \cdot,\cdot\right\rangle)$$ is isometric to the lattice $L:=(\mathbb{Z}^{\oplus 22}, -E_{8}\oplus-E_{8}\oplus U\oplus U\oplus U),$ where $$E_{8}$$ is the Cartan matrix of the corresponding root system and $$U$$ is the rank two hyperbolic matrix.

The holomorphic $$2$$-form $$\Omega$$ on $$X\ ,$$ which is unique up to scalars, spans a ray in $$H^2(X,\mathbb{Z})\otimes_\mathbb{Z}\mathbb{C}$$ and satisfies the well-known Riemann bilinear relation $\left\langle \Omega,\Omega\right\rangle =0,\,\,\mbox{ and }\,\,\left\langle \Omega,\overline{\Omega}\right\rangle >0.$ Thus after fixing a marking of $$X$$ that is an isomorphism $$(H^2(X,\mathbb{Z}) \left\langle \cdot,\cdot\right\rangle)\cong L\ ,$$ the period $$[\Omega]$$ lies in $\mathcal{D}=\{v\in \mathbb{P} L_\mathbb{C} \mid \left\langle v,v\right\rangle =0,\,\left\langle v, \bar v\right\rangle >0\}.$ This is the period domain for K3 surfaces, and the assignment $${X}\mapsto [\Omega]$$ for marked Kähler K3 surface $$X$$ is called the period map.

The Torelli problem is on how the element $$[\Omega]\in\mathcal{D}$$ determines the complex structure of a marked Kähler K3 surface. The global Torelli theorem for algebraic surfaces was proved by Pjateckii-Sapiro and Safarevi (Pjateckii-Sapiro and Safarevi, 1971); the Torelli theorem for Kähler K3 surfaces, proved by Burns and Rapoport (1975), and Looijenga and Peters (1980), states that the loci of the holomorphic two-form $$[\Omega]$$ in $$\mathcal{D}$$ uniquely determines the marked K3 surface $$X\ .$$

The surjectivity of the period map was proved by Kulikov (1977) followed by Persson-Pinkham (1981). The approach based on the Calabi-Yau metric was pioneered by Todorov (1980) and completed by Siu (1981) and Looijenga (1980). The proof relies heavily on Yau's solution of the Calabi conjecture. Later, based on Yau's solution of the Calabi conjecture, the main lemma of Burns and Rapoport (1975), and with the surjectivity of the period map of Kähler K3 surfaces, Siu (1983) proved that every K3 surface is Kähler.

### Moduli of high dimensional Calabi-Yau manifolds

The existence of the moduli of polarized Calabi-Yau manifolds was settled by the work of Viehweg (1995). The next question is the regularity of the moduli space. The first theorem was due to Bogomolov (1978) who proved that the universal deformation space of a compact Kähler-Hamiltonian manifold is unobstructed. In one of his unpublished manuscript, he also claimed that the same is true for any projective manifold with trivial first Chern class.

Todorov (1989) and Tian (1990) each confirmed this claim by proving that every Calabi-Yau manifold has unobstructed deformations. Both proofs used essentially the Calabi-Yau metric of the manifold to derive a differential-geometric computational equality that allows them to solve the Kuranishi equation in analytic deformation theory. This theorem is now referred to as the Bogomolov-Tian-Todorov unobstructedness theorem.

Ran (1992) and Kawamata (1992; 1995) gave new proofs of this unobstructedness result, based on the notion of $$T^1$$-lifting property. Their method was later applied to non-Kählerian Calabi-Yau manifolds and to some singular projective Calabi-Yau varieties.

On the moduli space, there is a natural Kähler metric obtained from the variation of the Ricci-flat metric called the Weil-Petersson metric. The volume of the moduli space of polarized Calabi-Yau manifolds with respect to this metric was proved by Lu and Sun (2004) and Todorov (2007) to be finite. This follows from finding a suitable metric that bounds the Weil-Petersson metric from above and satisfies the conditions of the Schwarz lemma (Yau, 1978), which implies generally that for any Hermitian metric defined on a quasi-projective manifold whose Ricci curvature has a strongly negative upper bound, the total volume is finite.

The study of the moduli space of complex structures on a Calabi-Yau threefold led Hitchin to study invariant functionals on differential forms (Hitchin, 2000). This approach is also useful when studying the associated flow equations that describes the geometry in terms of an evolving hypersurface. This approach has also led Hitchin (2001) to develop the geometry based on open orbits of $$GL\left( n,\mathbb{R}\right)$$ on $$k$$-forms, especially when $$k=3$$ (Hitchin, 2000). The structure of the moduli space of Calabi-Yau threefolds may also be related to the structure of those of Riemannian manifolds with exceptional holonomy in dimension 7 and 8.

As observed later, many examples of pairs of topologically distinct Calabi-Yau threefolds can be connected by flops or by small contractions followed by smoothing. For instance, Kawamata (2007) has recently proved that any two birational smooth Calabi-Yau manifolds can be connected by a sequence of flops. (Kawamata's general result is valid in any complex dimension. The proof for threefolds was given earlier by Kollár (1989) and fourfolds by Burns, Hu and Luo (2003).) One might speculate that the collection of all Calabi-Yau threefolds can be connected by such process. Reid raised this as his "fantasy" (1987).

For high dimensional Calabi-Yau manifolds, a major question is the Torelli problem. For hyperkähler manifolds, Huybrechts (1999) proved that the period map from the moduli space of marked hyperkähler manifolds to the period domain is surjective. The case of general Calabi-Yau manifolds has been recently studied by Liu-Sun-Todorov-Yau.

### The modularity of Calabi–Yau threefolds over $$\mathbb{Q}$$

In search of Calabi-Yau manifolds that distinguish themselves from the rest, their modularity become the focus of some researchers. This is interesting from the perspective of arithmetic geometry.

For a $$d$$-dimensional projective Calabi–Yau manifolds $$X$$ defined over $$\mathbb{Q}\ ,$$ it is said to be modular if the $$L$$-function of the Galois representation on the middle $$\ell$$-adic étale cohomology group $$H^d_{et}(X_{\bar{\mathbb{Q}}}, \mathbb{Q}_{\ell})$$ is equal to the product of $$L$$-functions of modular forms up to factors associated to bad primes. Part of the Langlands philosophy is the conjecture that all motives, in particular our $$X\ ,$$ are modular. When $$d$$ is even, $$H^d_{et}(X_{\bar{\mathbb{Q}}}, \mathbb{Q}_{\ell})$$ contains $$d/2$$-dimensional algebraic cycles, and the interesting part is the modularity of the sub-representation on the orthogonal complement of the images of algebraic cycles.

Calabi–Yau varieties of dimension $$1$$ are elliptic curves. The modularity of elliptic curves over $$\mathbb{Q}$$ has been established by Wiles (1995), and Taylor and Wiles (1995). They proved that the two-dimensional Galois representation associated to an elliptic curve over $$\mathbb{Q}$$ does come from a weight $$2=d+1$$ modular form.

Dimension $$2$$ Calabi–Yau varieties are K3 surfaces. For a K3 surface $$X$$ defined over $$\mathbb{Q}\ ,$$ $$H^2_{et}(X_{\bar{\mathbb{Q}}},\mathbb{Q}_{\ell})\ ,$$ which has dimension $$22\ ,$$ factors into a direct sum $$(NS(X)\oplus T(X))\otimes \mathbb{Q}_{\ell}$$ of the Néron-Severi group of algebraic cycles $$NS(X)$$ and the group of transcendental cycles $$T(X)\ .$$

There are partial result for small rank $$T(X)\ .$$ The lattice $$NS(X)$$ has rank at most $$20\ ;$$ when it is $$20\ ,$$ $$X$$ is called a singular K3 surface. (The term attractive K3 is sometimes also used in physics (Moore, 2007).) In this case, $$T(X)$$ has rank $$2$$ and defines a two-dimensional Galois sub-representation and the associated $$L$$-function $$L(T(X),s)\ .$$ The modularity of $$L(T(X),s)$$ has been established by Livné (1995) that $$L(T(X),s)$$ does come from a weight $$3=d+1$$ modular form of CM type. When $$T(X)$$ has rank $$3\ ,$$ its modularity follows from the modularity of elliptic curves, because $$T(X)$$ is endowed with an orthogonal pairing, so that it is essentially the symmetric square of a $$GL(2)$$ representation.

For Calabi-Yau threefolds defined over $$\mathbb{Q}\ ,$$ much is known for the rigid case, where there is no complex structure deformation. More specifically, we say that a Calabi-Yau threefold $$X$$ is rigid if the $$H^3_{et}(X_{\bar{\mathbb{Q}}}, \mathbb{Q}_{\ell})$$ has dimension $$2\ .$$ In this case, there is a two-dimensional Galois representation associated to $$X\ ;$$ the modularity has been established, under some mild conditions, that $$L(X,s)$$ is determined by some weight $$4=d+1$$ modular forms.

It is worth noticing that currently more than $$50$$ modular rigid Calabi-Yau threefolds over $$\mathbb{Q}$$ have been constructed, and the list is expanding.

The modularity question for a non-rigid Calabi-Yau threefold $$X$$ over $$\mathbb{Q}$$ poses more serious challenge as the dimension of $$H^3_{et}(X_{\bar{\mathbb{Q}}},\mathbb{Q}_{\ell})$$ gets larger. Much less is known.

An attractor flow equation on the complex structure moduli space of Calabi-Yau threefolds was found by Ferrara, Kallosh and Strominger (1995) in their study of BPS black holes solutions in string theory. Moore (2007) has shown that Calabi-Yau manifolds with complex structure located at an attractor fixed point on the moduli space exhibit interesting arithmetic properties.

## Calabi-Yau Manifolds in Physics

Calabi-Yau manifolds admit Kähler metrics with vanishing Ricci curvatures. They are solutions of the Einstein field equation with no matter. The theory of motions of loops inside a Calabi-Yau manifold provide a model of a conformal field theory. (It is called a sigma model in physics.) Because of this, Calabi-Yau manifolds are pivotal in superstring theory.

### Calabi-Yau manifolds in string theory

Superstring theory is a unified theory for all the forces of nature including quantum gravity. In superstring theory, the fundamental building block is an extended object, namely a string, whose vibrations would give rise to the particles encountered in nature. The constraints for the consistency of such a theory are extremely stringent. They require in particular that the theory takes place in a 10-dimensional space-time. To make contact with our 4-dimensional world, it is expected that the 10-dimensional space-time of string theory is locally the product $$M^{4}\times X$$ of a 4-dimensional Minkowski space $$M^{3,1}$$ with a 6-dimensional space $$X\ .$$ The 6-dimensional space $$X$$ would be tiny, which would explain why it has not been detected so far at the existing experimental energy levels. Each choice of the internal space $$X$$ leads to a different effective theory on the 4-dimensional Minkowski space $$M^{3,1}\ ,$$ which should be the theory describing our world.

It has long been argued that, in order to solve certain classic problems of unified gauge theories such as the gauge hierarchy problem, the 4-dimensional effective theory should admit an N=1 supersymmetry. In a fundamental paper, Candelas, Horowitz, Strominger and Witten (1985) analyzed what the constraint of that N=1 supersymmetry would mean for the geometry of the internal space $$X\ .$$ They found that, for the most basic product models with N=1 supersymmetry, the space $$X$$ must be a Calabi-Yau manifold of complex dimension 3. Shortly afterwards, Strominger (1986) considered slightly more general models, allowing warped products. For these models, the N=1 supersymmetry constraint results in a modification of the Ricci-flat equation of the earlier model.

### Calabi-Yau manifolds and mirror symmetry

Around 1987-1988, physicists including Dixon (1988), and Lerche, Vafa, and Warner (1989) observed that in mapping an abstract N=2 superconformal field theory to a possible geometrical realization as a Calabi-Yau sigma model, an ambiguity arose. A superconformal field theory has two natural rings (called (c,c) and (a,c) rings) as does a Calabi-Yau sigma model (the Dolbeault cohomology naturally splits into even and odd dimensional forms). The question which came to light was which of the two possible pairings of the conformal field theory and geometrical rings is induced by the map between the superconformal theory and the Calabi-Yau sigma model. Lerche, Vafa, and Warner conjectured that maybe both pairings are realized because, they suggested, Calabi-Yau threefolds come in pairs in which the even and odd cohomologies are interchanged. (To be precise, the interchange is between the Dolbeault cohomology $$H^{(p,q)}$$ with $$H^{(3-p,q)}$$ for Calabi-Yau threefolds.) At the time, the evidence in support of this conjecture was thin. Some suggested that a less radical solution to the observed ambiguity might be to keep the base Calabi-Yau manifold fixed and merely consider completing the geometrical model in two ways: by including its tangent bundle or its co-tangent bundle.

Nevertheless, in 1989, Greene and Plesser (Greene and Plesser, 1990), using the methods of conformal field theory as applied to Calabi-Yau sigma models realized as twisted products of $$N=2$$ minimal models, were able to establish that certain pairs of Calabi-Yau manifolds come in pairs in which their Hodge diamonds are mirror reflections (through a diagonal) of one another. Moreover, Greene and Plesser were able to establish that these pairs of Calabi-Yau manifolds, even though topologically distinct, when used as the basis for Calabi-Yau sigma models, give the same physical string theory. They named such pairs of Calabi-Yau manifolds mirror manifolds. The existence of such pairs of Calabi-Yau manifolds with specified properties are known to mathematicians as the Mirror Symmetry conjecture.

Of the few hundred mirror manifold pairs which Greene and Plesser's approach explicitly generated, the most famous example is the Fermat quintic $$X$$ in $$\mathbb{CP}^{4}$$ defined by the vanishing of $f\left( z_{0},z_{1},...,z_{4}\right) =z_{0}^{5}+z_{1}^{5}+\cdots+z_{4}^{5}+\psi\left( z_{0}z_{1}\cdots z_{4}\right)$ with its mirror being a crepant resolution of $$X/(\mathbb{Z}_5)^3\ ,$$ known as the mirror quintic.

Beyond constructing such pairs of mirror manifolds, Greene and Plesser noted that one implication of having a mirror pair yielding identical physical models is the existence of a highly nontrivial identity involving the so-called Yukawa couplings of each–quantities determined by the (quantum deformations of the) even cohomology ring of one manifold and the odd cohomology ring of its mirror.

A short time later, Candelas, de la Ossa, Green and Parkes (CDGP) (1992) studied the example of the mirror quintic pair by undertaking a detailed examination of the variation of Hodge structures of the mirror quintic. This work, interpreted mathematically by Morrison (1993) and Aspinwall and Morrison (1993), produced a beautiful solution to a long-standing problem in enumerative geometry – counting rational curves on a general quintic. It is important to note that this work relied on another conjecture – the mirror map conjecture – purporting to give the explicit map between the moduli spaces of this pair of Calabi-Yau mirror manifolds. The foundational discovery of Greene and Plesser, and of Candelas-de la Ossa-Green-Parkes, helped set in motion one of the most spectacular developments in modern mathematics.

A far-reaching generalization of the (genus zero) variation of Hodge structure – the so-called Kodaira-Spencer theory of gravity of Bershadsky, Cecotti, Ooguri and Vafa (BCOV) (1994) – later led to conjectural counting formulas for GW invariants of all genera for many Calabi-Yaus.

BCOV generalizes the variation of Hodge structure incorporating a natural hermitian structure which comes from the special Kähler geometry on the moduli space of Calabi-Yau manifolds. The generalized theory of Hodge structure is regarded as a special case of $$t$$-$$t^*$$ geometry" of two dimensional N=2 supersymmetric QFT due to Cecotti and Vafa (1993). At genus zero, $$t$$-$$t^*$$ geometry includes the successful associative relation, called Witten-Dijkgraaf-Verlinde-Verlinde (Witten, 1990; Dijkgraaf, Verlinde and Verlinde, 1991) (WDVV) equation, in quantum cohomology of projective manifolds. In 1993, BCOV (Bershadsky, Cecotti, Ooguri and Vafa, 1993) conjectured that one point function on a torus in the $$t$$-$$t^*$$ geometry of a Calabi-Yau manifold provides a non-trivial extension of the CDGP counting formula to genus one, called BCOV genus-one formula. Soon after, BCOV (Bershadsky, Cecotti, Ooguri and Vafa, 1994) introduced a certain recursion formula for higher genus $$(g\geq 2)$$ GW invariants, which is called the holomorphic anomaly equation. This recursion formula due to BCOV is still under extensive study for its mathematical ground.

A physical proof of mirror symmetry has been given by Hori and Vafa (2000). They demonstrated the equivalence at the level of two-dimensional gauged linear sigma model (Witten, 1993) which in the low-energy limit leads to the conformal field theory with Calabi-Yau manifold target space. (See also (Morrison and Plesser, 1996) for an earlier attempt along the same lines.)

### Mathematics inspired by mirror symmetry

One area inspired by the mirror symmetry conjecture is the construction of various enumerative invariants of Calabi-Yau manifolds. They appeared in the counting formula of CDGP. This development has led to a proof of the mirror symmetry conjecture for complete intersections by independent works: (Givental, 1996; Bini, De Concini, Polito and Procesi, 1998; Pandharipande, 1997; Lian, Liu and Yau, 1997).

In 1994, Kontsevich (1995) expanded and formulated his version of mirror symmetry as an equivalence between complex and symplectic geometry of Calabi-Yau manifolds in all dimension.

The geometric approach to mirror symmetry was finally unveiled by Strominger, Yau and Zaslow (SYZ) (1997) in their 1997 paper in which they proposed that mirror symmetry is a geometric version of the Fourier transformation along dual special Lagrangian tori fibrations on mirror Calabi-Yau manifolds. This SYZ proposal has guided many research works.

## Invariants of Calabi-Yau Manifolds

### Gromov-Witten Invariants

GW invariants are enumerative invariants that play an integral part of the Mirror Symmetry conjecture. The GW invariants were introduced by physicists for counting the holomorphic curves in Calabi-Yau threefolds which are needed to calculate worldsheet instanton corrections to the sigma model partiton function. In their paper, CDGP proposed a formula that counts the number of rational curves of fixed degree on a general quintic Calabi-Yau. (For Calabi-Yau manifolds, Mori theory of rational curves does not apply and it has only be shown by Heath-Brown and Wilson that Calabi-Yau manifolds with Picard number $$\rho >13$$ must have rational curves. See also (Wilson, 1989; Peternell, 1991).) Interpreting the CDGP work mathematically, Aspinwall and Morrison (1993) realized that the content of the CDGP formula were related to the work of Gromov (Gromov, 1985), who first introduced pseudoholomorphic curves to study symplectic geometry, and Witten’s work on two-dimensional topological topological field theory (Witten, 1988; Witten, 1991). Since then, many mathematicians have contributed to the mathematical foundation of this invariants.

There are two mathematical approaches to the problem - one based on symplectic geometry via pseudo-holomorphic maps to symplectic almost complex manifolds and the other based on algebraic geometry and the notion of stable maps. Both are dependent on understanding the gluing formula of WDVV (Dubrovin, 1992), which has been interpreted to be the associative law of quantum cohomology. The rigorous approach to pseudo-holomorphic maps which proved Gromov’s compactness theorem in full generality is due to Parker and Wolfson (1993) and R. Ye (1994) based on techniques developed by Sacks and Uhlenbeck (1981) and Siu and Yau (1980). Around 1994, Ruan and Tian (Ruan and Tian, 1995) used pseudo-holomorphic maps to define the symplectic GW invariants for all semipositive manifolds, which include Calabi-Yau manifolds. (The genus zero GW definition was also given by McDuff and Salamon (1994).) From the algebro-geometric perspective, Kontsevich and Manin (1994) also in 1994 gave an axiomatic treatment of GW classes and their properties for Fano varieties. Kontsevich, then introduced the notion of stable maps in algebraic geometry (Kontsevich, 1995). Using the moduli of stable maps, Li and Tian (1998) and Behrend and Fantechi (Behrend and Fantechi, 1997; Behrend, 1997) constructed the virtual cycles of DM-stack with perfect obstruction theories, thus constructing the GW invariants for all smooth projective varieties. The analytical framework for GW invariants was subsequently developed in full generality by (Fukaya and Ono, 1999; Li and Tian, 1998; Ruan, 2002; Siebert, 1999); some details were clarified later by Zinger (2004). The two approaches give identical invariants, confirmed by Li-Tian and Siebert (Li and Tian, 1999; Siebert, 1999).

### Counting formulas

In 1993-1995, Hosono-Klemm-Theisen-Yau (Hosono et al, 1995a; Hosono et al, 1995b) and Hosono-Lian-Yau (Hosono et al, 1996) made an interesting observation that was crucial to the understanding of the CDGP counting formula ($$g=0$$ mirror symmetry) and its generalizations. They observed that the Picard-Fuchs PDEs that compute the periods of a Calabi-Yau manifold have a "motivic" interpretation. Namely, in order for the classical Frobenius method to yield the periods, the Frobenius parameters must satisfy the cohomological relations on the mirror manifold exactly.

Using a combinatorial recipe of Batyrev and Borisov (Batyrev, 1994; Batyrev and Borisov, 1996) for constructing Calabi-Yau manifolds in toric varieties, Hosono-Klemm-Theisen- Yau (Hosono et al 1995a; Hosono et al, 1995b) and Hosono-Lian-Yau (Hosono et al, 1996) showed that this motivic relation holds true for all such Calabi-Yau manifolds, first for Calabi-Yau complete intersections in weighted pro jective spaces, then in toric varieties, and finally for noncompact Calabi-Yau manifolds which are sums of line bundles over toric varieties. This motivic relation allowed them to write down the counting formula for a Calabi-Yau manifold easily: it expresses the genus zero GW invariants of a Calabi-Yau manifolds explicitly in terms of the special geometry prepotential of the mirror manifold.

Independently, Candelas-de la Ossa-Font-Katz-Morrison (Candelas et al, 1994a; Candelas et al, 1994b) also generalized the CDGP work and gave detail analyses of models of Calabi-Yau hypersurfaces in weighted projected space with two Kähler parameters.

For positive genus, the Kodaira-Spencer theory of gravity of Bershadsky-Cecotti- Ooguri-Vafa (BCOV) (Bershadsky et al, 1994) has led to counting formulas for GW invariants of positive genera for many Calabi-Yau manifolds. Hosono-Lian-Yau (Hosono et al, 1996) generalized the BCOV genus-one formula to arbitrary Calabi-Yau complete intersections in toric varieties. Inspired by F-theory and M-theory, Klemm-Lian-Roan-Yau (Klemm et al, 1998) have also found a counting formula for GW invariants of Calabi-Yau manifolds of dimension 4 or higher.

More recently, based on the theory of BCOV and the geometry of the moduli of Calabi-Yau threefolds, Yamaguchi and Yau (2004) have shown that for the quintic threefold the topological partition functions of all genera can be expressed explicitly as polynomials of five known holomorphic functions. They conjectured that similar polynomials exist for all Calabi-Yau threefolds. As shown by Dijkgraaf (1995), the BCOV theory applied to an elliptic curve has a close similarity to the theory of quasi-modular forms of Kaneko and Zagier (1995). It has also allowed Huang-Klemm-Quackenbush (Huang et al, arXiv) to calculate the partition function up to genus $$51$$ for the quintic. The discovery by Yamaguchi and Yau has led to renewed interest on quasi-modular forms.

### Proofs of counting formulas for Calabi-Yau threefolds

In 1994, Kontsevich demonstrated (Kontsevich, 1995) that one can approach the GW-invariants of quintics by applying the Atiyah-Bott localization formula to the top Chern classes of vector bundles on the stable map moduli spaces of $$\mathbb{CP}^{4}\ .$$ Though his method in principle can determine the genus zero GW-invariants of all degrees, more insights are required to settle the mirror conjecture for quintics.

Two independent proofs of the CDGP formula used localization techniques in different ways. One approach based on quantum differential equation in the case of the quintics was due to Givental (1996), and was later expanded and clarified by others (Bini et al, arXiv; Pandharipande, 1998) in 1998. An independent approach based on functorial localization was given by Lian-Liu-Yau (Lian et al, 1997) in 1997; they later generalized their work to complete intersections in toric varieties in 1999. (See (Lian and Liu, arXiv) for a comparison of the two approaches.) The theory developed in (Lian et al, 1997; Lian et al, 1999; Lian et al, 2000) -- which is called the mirror principle -- have been applied to many other generalizations of the CDGP formula. By varying the possible K-classes and evaluating their Chern classes, their approach has also led to a number of new counting formulas for noncompact Calabi-Yau manifolds (Lian et al, 1997; Lian et al, 1999; Lian et al, 2000).

To prove the BCOV counting formula for higher genus GW invariants of quintics, a new localization formula for virtual fundamental classes had to be developed. This localization formula was worked out by Li and Zinger (Li and Zinger, arXiv) for complete intersection of projective spaces; the genus-one formula of BCOV for quintic Calabi-Yau was subsequently proved by Zinger (Zinger, arXiv).

### Integrability of mirror map and arithmetic applications

Mirror symmetry has many interesting and often unexpected connections and applications to mathematics. For instance, it is conjectured that near a certain large complex structure limit, the moduli space of a Calabi-Yau manifold admits certain special coordinates. Lian-Yau (Lian and Yau, arXiv) conjectured that the power series expansion of the mirror map in these coordinates always have integer coefficients. These expansions depend on some choices, but the integrality seems to be independent of such choices. This integrality conjecture has been proved for quintics and several other Calabi-Yau threefolds with $$h^{1,1}=1\ ,$$ and a number of isolated examples with $$h^{1,1}>1\ .$$

As Lian and Yau (1996) showed, mirror maps in some way can be thought of as generalization of modular functions. The precise conditions under which it is is a modular function were determined by Doran in (Doran, 2000). It is easy to see that the elliptic modular function $$j(\tau)$$ is nothing but the mirror map for elliptic curves. $$j(\tau)$$ satisfies a Schwarzian differential equation $$\{j(\tau),\tau\}=Q(j)\ ,$$ where $$Q(j)$$ is a certain rational function. And in fact, $$j$$ can be uniquely determined by the differential equation. For certain families of K3 surfaces, Clingher-Doran-Lewis-Whitcher (Clingher et al, 1880) derived the Schwarzian differential equation directly from geometry by studying the Picard-Fuchs equations over modular curves. Indeed, modularity of the mirror map implies integrality, and hence results for families of elliptic curves and K3 surfaces of generic Picard rank, 19. However, only a handful of specially constructed families of Calabi-Yau threefolds have classically modular mirror maps.

Klemm-Lian-Roan-Yau (Klemm et al, 1996) have also shown that mirror maps too satisfy similar, but higher order, nonlinear differential equations. These equations can be used to study divisibility property of the instanton numbers of Calabi-Yau threefolds. For example, it was shown that the instanton number $$n_{d}$$ predicted by the CDGP formula is divisible by 125 (at least for all $$d$$ coprime to $$5\ .$$) If $$n_{d}$$ correctly counts the number of smooth rational curves in a general quintic, as expected, then the divisibility property of $$n_{d}$$ above supports a conjecture of Clemens. On another front, the mirror principle, developed by Lian-Liu-Yau (Lian et al, 1997; Lian et al, 1999; Lian et al, 2000) also has important application in birational geometry. For example, Lee-Lin-Wang (Lee et al, arXiv) have used the mirror principle recently to study local models of Calabi-Yau manifolds in their study of analytic continuations of quantum cohomology rings under flops.

Arithmetic properties of algebraic Calabi-Yau manifolds defined over finite fields and their mirrors have been studied. Focusing on the one-parameter $$\psi$$ family of Fermat quintic threefolds $$X_\psi\ ,$$ Candelas, de la Ossa and Rodriguez-Villegas (Candelas et al, arXiv; Candelas et al, 2003) showed that the number of $$\mathbb{F}_{p}\,$$-rational points can be computed in terms of the periods of the holomorphic three-form. They also found a closed form for the congruence zeta function which counts the number $$N_r(X_\psi)$$ of $$\mathbb{F}_{p^r}\,$$-rational points. The zeta function is a rational function and the degrees of the numerator and denominator are exchanged between the zeta functions of $$X_\psi$$ and their mirror $$Y_\psi\ .$$ Interestingly, Wan (2006) has proved that $$N_r(X_\psi) = N_r(Y_\psi)~ ({\rm mod}~ p^r)$$ for arbitrary dimension Fermat Calabi-Yau manifolds and has conjectured that such relations should hold for all mirror pair Calabi-Yau manifolds in general.

### Donaldson-Thomas invariants

Another duality on Calabi-Yau threefolds is based on the invariants introduced by Donaldson and Thomas (1998). They introduced and studied the holomorphic Chern-Simons functional on the space of connections on a vector bundle over a Calabi-Yau threefold, defined by pairing the holomorphic 3-form with the Chern-Simons form. Their study leads to a collection of new invariants of Calabi-Yau threefolds, modulo some analytical technicality. These technicality can be by-passed in algebraic geometry using the moduli of stable sheaves and their virtual cycles.

A special case is the moduli of rank one stable sheaves. This leads to the virtual counting of ideal sheaves of curves, which are referred to as Donaldson-Thomas invariants. (These invariants based on ideal sheaves of curves can be generalized to all smooth threefolds.) In (Maulik et al, 2006), based on their explicit computation of such invariants for toric threefolds, Maulik-Nekrasov-Okounkov-Pandharipande (MNOP) conjectured that (the rank one version of) Donaldson-Thomas invariants is, in explicit form, equivalent to the GW invariants of the same varieties. Hence, assuming this conjecture, Donaldson-Thomas invariants provide {\it integers} underpinning the {\it rational} GW invariants.

Recently, Pandharipande and Thomas (Pandharipande and Thomas, arXiv) found a third curve-counting theory involving stable pairs. In order to define how to count these, one must think of curves as defining elements in the derived category of coherent sheaves, where they differ from the ideal sheaves of (Maulik et al, 2006) by a wall crossing in the space of stability conditions (Bridgeland, 2007). The more transparent geometry has made this curve-counting easier to study, leading to progress (Pandharipande and Thomas, arXiv) on a mathematical definition of the remarkable BPS invariants of Gopakumar-Vafa (Gopakumar and Vafa, arXiv), which give perhaps the best integer description of GW theory for threefolds.

The interaction of the MNOP duality with mirror symmetry is a little mysterious. It relates GW invariants, which belong to the A-model of mirror symmetry, to counting objects of the derived category (which describes the B-model) on the same manifold rather than its mirror. The point is that these latter invariants are independent of complex structures (they are deformation invariant), but depend on the stability conditions, one would hope that such invariants are symplectic invariants in nature, like GW invariants. A purely symplectic construction of the gauge-theoretic invariants of Donaldson-Thomas would be an important advance in our understanding. Mirror symmetry would then relate this derived category picture to the Fukaya category of the mirror. Counting stable sheaves gets replaced by counting special Lagrangians, as proposed by Joyce (2002). His counts are invariant under deformations of symplectic structures, but undergo wall crossings as the complex structure varies.

From physical considerations, Denef and Moore (Denef and Moore, arXiv) have independently found formulas describing the wall crossing phenomena in a special case. They are important for the counting of BPS D-branes bound states in string theory. Specifically, Donaldson-Thomas invariants have been identified with the counting of bound states of a single D6-brane with D2- and D0-branes. Wall crossings are also relevant for making precise the Ooguri-Strominger-Vafa conjecture (Ooguri et al, 2004) which relates the topological string partition function with BPS D-branes/black holes degeneracies. At the moment, wall crossing is a sub ject of much interest in both mathematics and physics, see for example (Kontsevich and Soibelman, arXiv; Gaiotto et al, arXiv). In particular, in the paper by Kontsevich and Soibelman, arXiv a general mathematical definition of the count of BPS states was suggested in the framework of 3-dimensional Calabi-Yau categories. Their wall-crossing formula contains as a special case most known wall-crossing formulas, including the one of Denef and Moore.

### Stable bundles and sheaves

Stable holomorphic bundles and sheaves are important geometric objects on Calabi-Yau manifolds and give interesting invariants (e.g. Donaldson-Thomas invariants). Stable principal G-bundles are also necessary data for heterotic strings on Calabi-Yau manifolds and for various duality relations in string theory. The stability condition of Mumford-Takemoto and of Gieseker on sheaves ensures that the moduli space is quasi-projective. By the results of Narasimhan and Seshadri (1965) for Riemann surfaces, and Donaldson (1985), Uhlenbeck and Yau (1986) for higher dimensions, there exist on stable (and poly-stable) bundles connections that solve the Hermitian-Yang-Mills equations. These equations are important for physical applications and requires that the (2,0) and (0,2) part of the curvature two-form vanish and the (1,1) part is traceless.

In dimension one, the classification of vector bundles on an elliptic curve was due to Atiyah (1957). The set of isomorphism classes of indecomposable bundles of a fixed rank and degree is isomorphic to the elliptic curve. For general structure groups, Looijenga (1976), and Bernstein and Shvartsman (1978) showed that the moduli space of semistable $$G$$ bundles for any simply-connected group $$G$$ of rank $$r$$ is a weighted projective space of dimension $$r\ .$$

In dimension two, Mukai (1984; 1987) studied in depth the moduli space $$\mathcal{M}^{H}(v)$$ of Gieseker-semistable sheaves $$F$$ on a smooth projective K3 surface $$(S,H)\ .$$ He showed that in case the moduli space $$\mathcal{M}^{H}(v)$$ is smooth, it is symplectic. His insight also led to the powerful Fourier-Mukai transformation.

Friedman-Morgan-Witten (Friedman et al, 1997; Friedman et al, 1998; Friedman et al, 1999) constructed stable principal G-bundles on elliptic Calabi-Yau threefolds (see also Donagi (1997), and Bershadsky-Johansson-Pantev-Sadov (Bershadsky et al, 1997).) The construction is based on spectral covers (Donagi and Markman, 1996) introduced on curves by Hitchin (1987a; 1987b). The spectral data consists of a hypersurface and a line bundle over it. The spectral cover construction can be interpreted in terms of a relative Fourier-Mukai transformation and have been used extensively in string theory (see, for example (Bouchard and Donagi, 2006; Braun et al, 2006; Andreas and Curio, 2007) and references therein).

Thomas (Thomas, 2000), Andreas, Hernández Ruipérez and Sánchez Gómez (Andreas and Hernández, arXiv) have constructed stable bundles on K3 fibration Calabi-Yau threefolds.

### Yau-Zaslow formula for K3 surfaces

In 1996, Yau and Zaslow (Yau and Zaslow, 1996) discovered a formula for the number of rational curves on K3 surfaces in terms of a quasi-modular form. Their method was inspired by string theory considerations.

Let $$X$$ be a K3 surface. Suppose $$C$$ is a holomorphic curve in $$X$$ representing a cohomology class $$\left[ C\right] \ .$$ We write its self-intersection number as $$\left[ C\right] \cdot\left[ C\right] =2d-2$$ and its divisibility, or index, as $$r\ .$$ If $$C$$ is a smooth curve, then $$d$$ is equal to the genus of $$C$$ and also to the dimension of the linear system of $$C\ .$$ If we denote the number of genus $$g$$ curves in $$X$$ representing $$\left[ C\right]$$ as $$N_{g}\left( d,r\right) \ ,$$ then the Yau-Zaslow formula says that when $$g=0$$ they are given by the following formula, $\sum_{d\geq0}N_{0}\left( d,r\right) q^{d}=\prod_{d\geq1}\left( \frac {1}{1-q^{d}}\right) ^{24}.$

The Yau-Zaslow formula was generalized by Göttsche (1998) to arbitrary projective surface. The universality for having such a formula for all surfaces was analyzed by Liu (2000) using Seiberg-Witten theory which is related to the curve counting problem by the work of Taubes on $$GW=SW\ .$$

The conjecture originated from a study by Yau and Zaslow on the BPS states in string theory on complex two dimensional Calabi-Yau manifolds, which are K3 surfaces. Shortly after the paper by Yau-Zaslow, Beauville (Beauville, 1999), and later Fantechi-Göttsche-van Straten (Fantechi et a, 1999), rephrased and clarified the argument of Yau-Zaslow in algebraic geometry for primitive class. Chen (X. Chen, 2002) proved that rational curves of primitive classes in general polarized K3 surfaces are nodal. Combined, these prove the Yau-Zaslow formula for primitive classes.

The Yau-Zaslow formula is for all index $$r\geq 1\ .$$ Following the original approach of Yau-Zaslow, Li and Wu (2006) proved the conjecture for non-primitive classes of index at most five under the assumption that all rational curves are nodal.

Via a different approach, Bryan and Leung (2000) proved the formula for the primitive case by considering elliptic K3 surfaces with section by computing the family GW invariants for the twistor family. These invariants are typically difficult to compute and they used a clever matching method to transport it to an enumerative problem for rational surfaces and then used Cremona transformations to further simplify it. Their method is more powerful than the sheaf-theoretic approach in that it works for any genus as well.

Using a degeneration for the family GW invariants, J.H. Lee-Leung settled the $$r=2$$ case of the Yau-Zaslow formula (Lee and Leung, 2005) and the genus one formula (Lee and Leung, 2006).

Recently Klemm, Maulik, Pandharipande and Scheidegger (Klemm et al, 2008) proved the Yau-Zaslow formula for any classes by studying a particular Calabi-Yau threefold $$M$$ with a K3 fibration. The Yau-Zaslow number can be related to the GW invariants on $$M$$ representing fiber classes. Using localization techniques to compute these threefold invariants they proved the Yau-Zaslow formula.

### Chern-Simons knot invariants, open strings and string dualities

Calabi-Yau geometry is the central object in string duality to unify different types of string theory. Mirror symmetry is just the duality between IIA and IIB string theory as discussed above. Using string duality between the large $$N$$ Chern-Simons theory and the topological string theory of non-compact toric Calabi-Yau manifolds, string theorists have made many striking conjectures about the moduli spaces of Riemann surfaces, Chern-Simons knot invariants and GW invariants. Of note are two which have been rigorously proven. First, the Mariño-Vafa conjecture (2002) which expresses the generating series of triple Hodge integrals on moduli spaces of Riemann surfaces for all genera and any number of marked points in terms of the Chern-Simons knot invariants was proved by C.-C. Liu-K. Liu-Zhou in (Liu et al, 2004). Second, the Labastilda-Mariño-Ooguri-Vafa conjecture (Ooguri and Vafa, 2000; Labastida and Mariño, 2000; Labastida and Mariño, 2002).

GW invariants for all genera and all degrees can be explicitly computed for non-compact toric Calabi-Yau manifolds via the theory of topological vertex. In (Aganagic et al, 2005 ), Aganagic, Klemm, Mariño and Vafa proposed a theory to compute GW invariants in all genera and all degrees of any smooth non-compact toric Calabi-Yau threefold. In that paper, they first postulated the existence of open GW invariants that count holomorphic maps from bordered Riemann surfaces to $$\mathbb{C}^{3}$$ with boundaries mapped to Lagrangian submanifolds, which they called the topological vertex; they then argued based on a physically derived duality between Chern-Simons theory and GW theory that the topological vertex can be expressed in terms of the explicitly computable Chern-Simons link invariants. Then by a gluing algorithm, they derived an algorithm computing all genera GW invariants of toric Calabi-Yau threefolds.

In (Li et al, arXiv), J. Li, C.-C. Liu, K. Liu and J. Zhou (LLLZ) developed the mathematical theory of the open GW invariants for toric Calabi-Yau threefold. (In the case compact Calabi-Yau threefolds, open GW invariants have only been defined in the case where the Lagrangian submanifold is the fixed point set of an antiholomorphic involution (Solomon, arXiv). See (Walcher, 2007; Pandharipande et al, 2008) for calculations of open GW invariants on the Calabi-Yau quintic.) The definition of LLLZ relies on applying the relative GW invariants of J. Li (2001; 2002) to formal toric Calabi-Yau threefolds. By degenerating a formal toric Calabi-Yau to a union of simple ones, they derived an algorithm that expresses the open GW invariants of any (formal) toric Calabi-Yau in terms of that of the simple one. Their results express the open GW invariants in terms of explicit combinatorial invariants related to the Chern-Simons invariants. In many cases their combinatorial expressions coincide with those of (Aganagic et al, 2005 ), and they conjectured that the two combinatorial expressions should be equal in general. Later, a proof of this conjecture appeared in the work of Maulik-Oblomkov-Okounkov-Pandharipande (Maulik et al, arXiv) as part of the proof of the famous conjecture GW=DT". Combined, all genera GW invariant for toric Calabi-Yau threefolds is solved. By using the results of (Li et al, arXiv), Peng (2007) was able to prove the integrality conjecture of Gopakumar-Vafa for all formal toric Calabi-Yau manifolds.

When applying the mirror principle to certain toric Calabi-Yau manifolds, we get the local mirror formulas of Chiang-Klemm-Yau-Zaslow (Chiang et al, arXiv) which are closely related to geometric engineering in string theory (Katz et al, 1997a). This is an important technique to recover gauge theory such as the Seiberg-Witten theory at various singularities in the moduli space of string theory (Katz et al, 1997b). Chiang-Klemm-Yau-Zaslow (Chiang et al, arXiv) also studied the asymptotic growth of genus zero Gromov-Witten invariants as the degree runs to infinity. Computational evidences have suggested in many cases a relationship between these growth rates and special values of L-functions. These observations have now been geometrically explained by Doran-Kerr (Doran and Kerr, arXiv), who showed, using higher Abel-Jacobi maps, that they follow from the deep mathematical conjectures of Beilinson-Hodge and Beilinson-Bloch.

## Homological Mirror Symmetry

The Homological Mirror Symmetry (HMS) conjecture was made in 1994 by Maxim Kontsevich (Kontsevich, 1995). This was a proposal to give an explanation for the phenomena of mirror symmetry. This conjecture, very roughly, can be explained as follows. Let X and Y be a mirror pair of Calabi-Yau manifolds. We view X as a complex manifold and Y as a symplectic manifold. The idea is that mirror symmetry provides an isomorphism between certain aspects of complex geometry on X and certain aspects of symplectic geometry on Y.

More precisely, Kontsevich suggested that the bounded derived category of coherent sheaves on X is isomorphic to the bounded derived of the Fukaya category of Y. The first object has been well-studied, and is known to capture a significant amount of information about the complex geometry on X, while the Fukaya category is a much less familiar object introduced by Fukaya (1993). This is not a true category, but something known as an $$A_{\infty}$$ category: the composition of morphisms is not associative, but only associative up to homotopy. The Fukaya category captures information about the symplectic geometry of Y. Its objects are Lagrangian submanifolds of Y and morphisms come from intersection points of Lagrangian submanifolds. Compositions involve counting holomorphic disks, and essentially arise from the product in Floer homology.

The homological mirror symmetry conjecture has remained an imposing problem. There have been a number of different threads of work devoted to this. Work of a number of researchers, especially Polishchuk and Zaslow (1998), Fukaya (2002), Kontsevich and Soibelman (2000) dealt with the simplest cases, namely mirror symmetry for elliptic curves and abelian varieties, respectively. Other work has been devoted to clarifying the conjecture: at first sight, the two categories cannot be isomorphic since the derived category is an actual triangulated category, while the Fukaya category is not an actual category and is not likely to be triangulated. There are various ways around these issues, and there are now precise rigorous statements. Most significantly, the work of Seidel (Seidel, arXiv) has proved the conjecture for quartic surfaces in projective three-space. Also, in the cited paper by Kontsevich and Soibelman a general approach to the proof of HMS was suggested based on their modification of the SYZ conjecture (see below) and the ideas of (non existent at the time) tropical geometry. This strengthens the original idea implicit in SYZ and made explicit by Leung, Yau, and Zaslow (Leung et al, 2000), that the mirror functor is defined as a real Fourier-Mukai transform.

The HMS conjecture implies that complex manifolds which have equivalent bounded derived categories are mirrored to the same manifold. These manifolds, related by Fourier-Mukai transforms, are called Fourier-Mukai partners. Orlov (1997) has determined both the group of autoequivalences and the Fourier-Mukai partners of an abelian variety. Interesting results have also known for K3 surfaces. Mukai (1984) long ago showed that any Fourier-Mukai partner of a given K3 surface is again a K3. The Fourier-Mukai transform induces a Hodge isometry of the “Mukai lattice” of K3 (Orlov, arXiv). Bridgeland and Maciocia (2001) have shown that the number of Fourier-Mukai partners of any given K3 is finite. Hosono, Lian, Oguiso, and Yau (Hosono et al, 2004b) have recently, given an explicit counting formula for this number. A similar formula was given for abelian surfaces and was used to answer an old question of T. Shioda (Hosono et al, 2003a). They have also given a description for the group of autoequivalences of the bounded derived category of a K3 surface (Hosono et al, 2004a). It turns out that the Fourier-Mukai number formula is closely related to the class numbers of imaginary quadratic fields of prime discriminants (Hosono et al, 2004b). There is also a nice analogue for real quadratic fields. As shown in (Hosono et al, 2003b), the real case turns out to be crucial for classifying $$c=2$$ rational toroidal conformal field theory in physics.

The HMS conjecture for Calabi-Yau manifolds has been generalized to Fano varieties. For toric varieties, the work of Abouzaid (Abouzaid, arXiv) established part of the conjecture and was recently settled by Fang-Liu-Treumann-Zaslow (Fang et al, arXiv). Moreover, for surfaces, Auroux-Katzarkov-Orlov (Auroux et al, 2006; Auroux et al, 2008) have proved the HMS conjecture for some toric surfaces (i.e. weighted projective planes, Hirzebruch surfaces, and toric blowups of $$\mathbb{P}^2$$) and also non-toric del Pezzo surfaces.

Another thread has been addressing the question of how more traditional aspects of mirror symmetry, such as holomorphic curve counting, would follow from homological mirror symmetry.

## SYZ Geometric Interpretation of Mirror Symmetry

### Special Lagrangian submanifolds in Calabi-Yau manifolds

By the Wirtinger formula for Kähler manifolds, every complex submanifold in $$X$$ is absolutely volume minimizing. This is a special case of calibration, a notion introduced by Harvey and Lawson (Harvey and Lawson, 1982) in analyzing area-minimizing subvarieties, and later on rediscovered in physics by Becker-Becker-Strominger (Becker et al, 1995) from supersymmetry considerations. Special Lagrangian submanifolds in Calabi-Yau manifolds form another class of examples of calibrated submanifolds. A real $$n$$-dimensional submanifold $$L$$ in $$X$$ is called special Lagrangian if the restrictions of both $$\omega$$ and $$\operatorname{Im}\,\Omega$$ to $$L$$ are zero: $\omega|_{L}=\operatorname{Im}\,\Omega|_{L}=0\text{.}$ As calibrated submanifolds, special Lagrangian submanifolds are always absolutely volume minimizing.

### The SYZ conjecture - SYZ transformation

In string theory, each Calabi-Yau threefold $$X$$ determines two twisted theories, one A-model and another B-model. The mirror symmetry between $$X$$ and its mirror $$Y$$ interchanges the two models between them. From the mathematical perspective, A-model is about the symplectic geometry of $$X$$ and B-model is about the complex geometry of $$Y\ .$$ $\begin{array} [c]{ccc} \begin{array} [c]{c} \text{A-model on }X\\ \text{(symplectic geometry)} \end{array} & \overset{}{\overleftrightarrow{\text{mirror symmetry}}} & \begin{array} [c]{c} \text{B-model on }Y\\ \text{(complex geometry)} \end{array} \end{array}$ The search for the underlying geometric root of this symmetry led Strominger, Yau and Zaslow to their conjecture.

In 1996, Strominger, Yau and Zaslow (Strominger et al, 1996) proposed that for a mirror pair $$(X,Y)$$ that is near a large volume/complex structure limit,

(1) both admit special Lagrangian torus fibrations with sections: $\begin{array} [c]{ccc} T & \overleftrightarrow{\text{dual tori}} & T^{\ast}\\ \downarrow & & \downarrow\\ X & & Y\\ \downarrow & & \downarrow\\ B & & B^{\ast} \end{array}$

(2) the two torus fibrations are dual to each other;

(3) a fiberwise Fourier-Mukai transformation along fibers interchanges the symplectic (resp. complex) geometry on $$X$$ with the complex (resp. symplectic) geometry on $$Y\ .$$

This is called the SYZ mirror transformation.

On the nutshell, it says that the mysterious mirror symmetry is simply a Fourier transform. The quantum corrections, for instance the GW invariants, come from the higher Fourier modes. The SYZ conjecture inspired a flourish of work to understand mirror symmetry, which include works of Gross (and with Siebert) (Gross, 2001a; Gross, 2001b; Gross, 2001c; Gross and Siebert, 2003; Gross and Siebert, arXiv), Joyce (2001; 2003), Kontsevich and Soibelman (2001; 2006), Vafa (Vafa, arXiv), Leung-Yau-Zaslow (Leung et al, 2000) and many others. On the other hand, it has led to new developments of other branches of mathematics, including the calibrated geometry of special Lagrangian submanifolds and the affine geometry with singularities. The work of Auroux has shed some lights on the phenomenon of quantum corrections (Atiyah, arXiv).

### Special Lagrangian geometry

Special Lagrangian submanifolds coupled with unitary flat bundles are branes in A-model in string theory. These geometric objects are crucial to the understanding of the SYZ conjecture. So far, many examples were constructed using cohomogeneity one method by Joyce (2001), using singular perturbation method by Butscher (2004), Lee (2003), Haskins and Kapouleas (2007) and others. Their deformations are studied by McLean (1998); their moduli spaces by Hitchin (1997); their existence by Schoen and Wolfson (1999) using variational approach and by Smoczyk and M.-T. Wang (2002) using mean curvature ﬂow. Thomas and Yau (2002) formulated a conjecture on the existence and uniqueness of special Lagrangian submanifolds which is the mirror of the theorem of Donaldson, Uhlenbeck and Yau (Donaldson, 1985; Uhlenbeck and Yau, 1986) of the existence of unique Hermitian Yang-Mills connection on any stable holomorphic vector bundle.

### Special Lagrangian fibrations

SYZ conjecture predicts that mirror Calabi-Yau manifolds should admit dual torus fibrations whose fibers are special Lagrangian submanifolds, possibly with singularities.

Lagrangian fibrations is an important notion in symplectic geometry as real polarizations, as well as in dynamical system as completely integrable systems. Their smooth fibers admit canonical integral affine structures and therefore they must be tori in the compact situation. Toric varieties $$\mathbb{P}_{\Delta},$$ for instance $$\mathbb{CP}^{n+1}\ ,$$ are examples of Lagrangian fibrations in which the fibers are orbits of an Hamiltonian torus action and the base is a convex polytope $$\Delta\ .$$

A complex hypersurface $$X=\left\{ f=0\right\}$$ in $$\mathbb{CP}^{n+1}$$ is a Calabi-Yau manifold if $$\deg f=n+2\ .$$ The most singular ones is when $$X$$ is a union of coordinate hyperplanes in $$\mathbb{CP}^{n+1}\ ,$$ which is an example of the large complex structure limit. Such limiting points on the moduli space are important and an explicit construction of them for Calabi-Yau toric hypersurfaces as $$T$$-fixed points on the moduli space has been given by Hosono-Lian-Yau (Hosono et al, 1997). A numerical criterion for the large complex structure limit in any one parameter family of Calabi-Yau manifolds has also been given by Lian-Todorov-Yau (Lian et al, 2005). At this most singular limit, $$X$$ inherits a torus fibration from the toric structure on $$\mathbb{CP}^{n+1}\ .$$ Thus one can try to perturb this to obtain Lagrangian fibration structures on nearby smooth Calabi-Yau manifolds. This approach was carried out by Gross (2001c), Mikhalkin (2004), Ruan (1999; 2002) and Zharkov (2000). This approach can be generalized to Calabi-Yau hypersurfaces $$X$$ in any Fano toric variety $$\mathbb{P}_{\Delta}\ .$$ Furthermore, their mirror manifolds $$Y$$ are Calabi-Yau hypersurfaces in another Fano toric variety $$\mathbb{P}_{\nabla}$$ whose defining polytope is the polar dual to $$\Delta\ .$$

The situation is quite different for Calabi-Yau twofolds, namely K3 surfaces, or more generally for hyperkähler manifolds. In this case, the Calabi-Yau metric on $$X$$ is Kähler with respect to three complex structures $$I\ ,$$ $$J$$ and $$K\ .$$ When $$X$$ admits a $$J$$-holomorphic Lagrangian fibration, then this fibration is a special Lagrangian fibration with respect to the Kähler metric $$\omega_{I}\ ,$$ as well as $$\omega_{K}\ .$$ Furthermore, SYZ also predicts that mirror symmetry is merely a twistor rotation from $$I$$ to $$K$$ in this case. There are plenty of K3 surfaces which admit elliptic fibrations and they are automatically complex Lagrangian fibrations because of their low dimension. Furthermore Gross and Wilson (2000) described the Calabi-Yau metrics for generic elliptic K3 surfaces by using the singular perturbation method. They used model metrics which were constructed by Greene, Shapere, Vafa and Yau (Greene et al, 1990) away from singular fibers and by Ooguri and Vafa (1996) near singular fibers.

### The SYZ transformation

Recall that SYZ conjecture says that mirror symmetry is a Fourier-Mukai transformation along dual special Lagrangian torus fibrations. We also need to include a Legendre transformation on the base affine manifolds. This SYZ transformation was generalized to the mirror symmetry for local Calabi-Yau manifolds by Leung and Vafa (1998).

On the mathematical side, Leung-Yau-Zaslow (Leung et al, 2000) and Leung (2005) used the SYZ transformation to verify various correspondences between symplectic geometry and complex geometry between semi-flat Calabi-Yau manifolds when there is no quantum corrections. To include quantum corrections in the SYZ transformation for Calabi-Yau manifolds is a more difficult problem. In the Fano case, there are recent results on applying the SYZ transformation with quantum corrections by Auroux (Atiyah, arXiv), Chan-Leung (Chan and Leung, arXiv) and Fang (Fang, arXiv).

### The SYZ conjecture and tropical geometry

Work of Joyce (2003) forced a rethinking of the SYZ conjecture in a limiting setting. The SYZ mirror transformation is now believed to be applicable near the large complex structure limit points. Two groups of researchers, Gross and Wilson (2003) on the one hand and Kontsevich and Soibelman (2001) on the other, suggested that near a large complex structure limit of n-dimensional Calabi-Yau manifolds, the Ricci-flat metric on the Calabi-Yau manifold converges (in a precise sense known as Gromov-Hausdorff convergence) to an topological manifold endowed with integral affine structure and (singular on the codimension two submanifold) metric, satisfying real Monge-Ampere equation.

The reduction of the Kahler-Einstein metric by the flat-tori action creates a metric on the quotient space compatible with the affine structure defined there. This was first introduced by Cheng and Yau in 1980 (during the Beijing Symposium on Differential Geometry and Differential Equations) where they called a class of affine manifold that admits a metric given by the Hessian of a smooth function to be affine-Kahler. The volume form defined by the metric is an affine volume form, similar to the one given by the Calabi-Yau manifolds. Cheng and Yau proved that such metrics must admit a singularity if the manifold is not a flat torus. There is a question of what the orbit space of this singular fibration should be. In the Calabi-Yau case, it was conjectured to be a topological n-sphere. For example, in the simplest case of an elliptic curve (a real two-dimensional torus), the torus gets thinner as the large complex structure limit is approached, until it converges to a circle. Therefore, the idea is that in the large complex structure limit, the SYZ fibration is expected to be better behaved though the fibers of the SYZ fibration will collapse, with its volume going to zero in the limit.

In any event, once one has this picture of a collapsing fibration, one can ask for a description of the behavior of holomorphic curves in the fibration as the fibres collapse. The expectation is that a holomorphic curve converges to a piecewise linear graph on the limiting sphere. This graph should satisfy certain conditions which turn this graph into what is now known as a "tropical curve". This terminology arises from the tropical semiring, which is the semiring consisting of real numbers, with addition given by maximum and multiplication given by the usual addition. Tropical varieties are then defined by polynomials over the tropical semiring, and the "zeroes" of a tropical polynomial are in fact points where the piecewise linear function defined by the tropical polynomial is not smooth. This gives rise to piecewise linear varieties, and tropical curves arising as limits of holomorphic curves are examples of such.

This picture began to emerge in the works of Fukaya (2005), Kontsevich and Soibelman (2001) around 2000. In particular, Kontsevich’s suggestion that one could count holomorphic curves by counting tropical curves was realized in 2003 by Mikhalkin (Mikhalkin, 2005), when he showed that curves in toric surfaces could be counted using tropical geometry.

For the purposes of mirror symmetry, it is then important to understand how tropical geometry arises on the mirror side. The initial not so rigorous work of Fukaya in 2000 gave some suggestions as to how this might happen in two dimensions. This was followed by the work of Kontsevich and Soibelman (Kontsevich and Soibelman, 2006) in 2004, again in two dimensions, and the work of Gross and Siebert (Gross and Siebert, arXiv) in 2007 in all dimensions, which demonstrate that the geometry of Calabi-Yau manifolds near large complex structure limits can be described in terms of data of a tropical nature. This provides the clearest link to date between the two sides of mirror symmetry. Another confirmation of this idea emerged from a very general approach to mirror symmetry suggested by Kontsevich and Soibelman (2001). It is based on the idea of collapsing Conformal Field Theories. From the point of view of that paper there is an analog of the tropical limit for a wide class of degenerating CFTs. When a family of CFTs correspond to a maximally degenerating family of Calabi-Yau manifolds then the tropical limit can be described in terms of the Gromov-Hausdorff collapse.

## Geometries Related to Calabi-Yau Manifolds

### Non-Kähler Calabi-Yau manifolds

Given a smooth three dimensional complex manifold $$X$$ with trivial canonical line bundle, i.e. $$K_{X}\cong O_{X}\ .$$ When $$X$$ is Kähler, Yau's theorem (1979) provides a unique Ricci-flat Kähler metric in each Kähler class.

A large class of such threefolds which are non-Kähler are obtained by Clemens (1983) and Friedman (1986) from Calabi-Yau threefolds by an operation called extremal transition or its inverse. An extremal transition is a composition of blowing down rational curves and smoothing the resulting singularity. It has the effect of decreasing the dimension of $$H^{2}\left( X,\mathbb{R}\right)$$ and increasing the dimension of $$H^{3}\left( X,\mathbb{R}\right)$$ while keeping their sum fixed. For example, the connected sum of $$k$$ copies of $$S^{3}\times S^{3}$$ for any $$k\geq2$$ can be given a complex structure in this way. Based on this construction, Reid (1987) speculated that any two Calabi-Yau threefolds are related by deformations, extremal transitions and their inverses, even though their topologies are different. This speculation demonstrates the potential role of non-Kähler complex manifolds.

It is important to construct canonical metrics on such non-Kähler manifolds which are counterparts of Ricci-flat Kähler metrics on Calabi-Yau manifolds. In 1986, Strominger proposed for supersymmetric compactification in the theory of heterotic string a system of a pair $$(\omega,h)$$ of a Hermitian metric $$\omega$$ on a complex three-dimensional manifold $$X$$ with a non-vanishing holomorphic three form $$\Omega$$ and a Hermitian metric $$h$$ on a vector bundle $$V$$ on $$X\ .$$ The Strominger system is such a pair satisfying the elliptic system of differential equations, $d(\| \Omega\|_\omega\, \omega^{2})=0,$ $F\wedge\omega^{2}=0,\ \ F^{2,0}=F^{0,2}=0,$ $4\sqrt{-1}\partial\bar{\partial}\omega=\alpha^{\prime}\left( Tr_{T_X}\left( R^{2}\right) -Tr_{E}\left( F^{2}\right) \right) ,$ where $$R$$ (resp. $$F$$) is the curvature of $$\omega$$ (resp. $$h$$). The first equation is equivalent to the existence of a balanced metric, also the same as the existence of supersymmetry. The system of equations in the second line is the Hermitian-Yang-Mills equations.

When $$V$$ is the tangent bundle $$T_{X}$$ and $$\omega$$ is Kähler, the system is solved by the Calabi-Yau metric. Using perturbation method, J. Li and S.-T. Yau (2005) constructed smooth solutions to a class of Kähler Calabi-Yau with irreducible solutions for vector bundles with gauge group $$SU\left( 4\right)$$ and $$SU\left( 5\right) \ .$$

The first existence result for solutions of Strominger system for a non-Kähler Calabi-Yau was due to Fu-Yau on a class of torus bundles over K3 surfaces (Fu and Yau, 2008; Becker et al, 2006). (The construction of the complex structure is called the Calabi-Eckmann construction (Calabi and Eckmann, 1953) and was carried out by Goldstein-Prokushkin (Goldstein and Prokushkin, 2004). Based on physical arguments of superstring dualities, the existence of such solutions was suggested in (Dasgupta et al, 1999; Becker and Dasgupta, 2002).) Mathematical construction of such "balanced" metrics on manifolds constructed by Clemens-Friedman was recently carried out rigorously by Fu-Li-Yau (Fu et al, arXiv)(Note that these are not the same as the metrics considered in Subsection 1.8 above.)

### Symplectic Calabi-Yau manifolds

Another generalization of Calabi-Yau manifolds are symplectic Calabi-Yau manifolds. Recall a symplectic manifold $$(X,\omega)$$ is an even dimensional (real) manifold $$X$$ with $$\omega$$ a closed, non-degenerate $$2$$-form on $$X\ .$$ Examples of symplectic manifolds include Kähler manifolds. Using any compatible almost complex structure on $$X\ ,$$ we can define the first Chern class $$c_{1}\left( X\right)$$ for any symplectic manifold $$X\ .$$

Symplectic Calabi-Yau manifolds are symplectic manifolds with $$c_{1}\left( X\right) =0\ .$$ In dimension four, we have the Kodaira-Thurston examples; the homological type of such symplectic manifolds are classified, due to the work of T.-J. Li (2006), and to Bauer (2008), that their Betti numbers are in the range $$b_{1} \leq4\ ,$$ $$b_2^{+}\leq3$$ and $$b_2^{-}\leq19\ .$$ To their smooth structures, it is conjectured that the diffeomorphism types of such manifolds are either Kähler surfaces with zero Kodaira dimension or oriented torus bundles over torus.

In higher dimensions, Smith-Thomas-Yau (Smith et al, 2002) has constructed many such examples of symplectic Calabi-Yau manifolds. They contain structures which are mirror to complex non-Kähler Calabi-Yau structures on connected sums of $$S^{3}\times S^{3}\ .$$ As described in (Smith et al, 2002), the symplectic mirror of the Clemens-Friedman construction reverses the conifold transition by first collapsing Lagrangian three-spheres and then replacing them by symplectic two-spheres. If one can collapse all three-spheres, then such a process should result in symplectic Calabi-Yau structures on connected sums of $$\mathbb{CP}^{3}\ .$$

As the Strominger-Fu-Yau geometry on complex non-Kähler Calabi-Yau manifolds plays an important role in string theory, it is expected to have a dual system on these symplectic Calabi-Yau manifolds which will also play an important role in string theory.

One can also generalize the Ricci-flat condition in dimension four. Donaldson conjectured in (Donaldson, 2006) that an analogue of the Calabi-Yau theorem should hold on symplectic 4-manifolds. If it is true, there are interesting applications to symplectic topology in dimension four. So far relatively little is known about this conjecture, but some progress has been made in (Weinkove, 2007) and (Tosatti et al, 2008). There it is shown that the conjecture holds when the manifold is nonnegatively curved, so for example on $$\mathbb{CP}^{2}$$ with a small perturbation of the standard Kähler structure.

Note by Yau: In writing an overview of such a broad subject area, the need to be inclusive was recognized and many experts were consulted. But unfortunately, in the citing of original references and the topics covered, omissions inevitably always occur, and for this, sincere apology is offered.