Topological recursion
In mathematics, topological recursion is a recursive definition of invariants of spectral curves.
It has applications in enumerative geometry, random matrix theory, mathematical physics, string theory, knot theory.

Introduction 
The topological recursion is a construction in algebraic geometry. It takes as initial data a spectral curve: the data of , where:  is a covering of Riemann surfaces with ramification points;  is a meromorphic differential 1-form on , regular at the ramification points;  is a symmetric meromorphic bilinear differential form on  having a double pole on the diagonal and no residue.

The topological recursion is then a recursive definition of infinite sequences of symmetric meromorphic n-forms  on , with poles at ramification points only, for integers g≥0 such that 2g-2+n>0. The definition is a recursion on the integer 2g-2+n.

In many applications, the n-form  is interpreted as a generating function that measures a set of surfaces of genus g and with n boundaries. The recursion is on 2-2g+n the Euler characteristics, whence the name "topological recursion".

Origin 
The topological recursion was first discovered in random matrices. One main goal of random matrix theory, is to find the large size asymptotic expansion of n-point correlation functions, and in some suitable cases, the asymptotic expansion takes the form of a power series. The n-form   is then the gth coefficient in the asymptotic expansion of the n-point correlation function. It was found that the coefficients  always obey a same recursion on 2g-2+n. The idea to consider this universal recursion relation beyond random matrix theory, and to promote it as a definition of algebraic curves invariants, occurred in Eynard-Orantin 2007 who studied the main properties of those invariants.

An important application of topological recursion was to Gromov–Witten invariants. Marino and BKMP conjectured that Gromov–Witten invariants of a toric Calabi–Yau 3-fold  are the TR invariants of a spectral curve that is the mirror of .

Since then, topological recursion has generated a lot of activity in particular in enumerative geometry.
The link to Givental formalism and Frobenius manifolds has been established.

Definition 

(Case of simple branch points. For higher order branchpoints, see the section Higher order ramifications below)

  For  and :

where  is called the recursion kernel:

and  is the local Galois involution near a branch point , it is such that .
The primed sum  means excluding the two terms  and .

 For  and :

with  any antiderivative of .

 The definition of  and  is more involved and can be found in the original article of Eynard-Orantin.

Main properties 

 Symmetry: each  is a symmetric -form on .
 poles: each  is meromorphic, it has poles only at branchpoints, with vanishing residues.
 Homogeneity:  is homogeneous of degree . Under the change , we have  .
 Dilaton equation:
 where .

 Loop equations: The following forms have no poles at branchpoints

where the sum has no prime, i.e. no term excluded.

 Deformations: The  satisfy deformation equations
 Limits: given a family of spectral curves , whose limit as  is a singular curve, resolved by rescaling by a power of , then .
 Symplectic invariance: In the case where  is a compact algebraic curve with a marking of a symplectic basis of cycles,  is meromorphic and  is meromorphic and   is the fundamental second kind differential normalized on the marking, then the spectral curve  and , have the same  shifted by some terms.
 Modular properties: In the case where  is a compact algebraic curve with a marking of a symplectic basis of cycles,  and   is the fundamental second kind differential normalized on the marking, then the invariants  are quasi-modular forms under the modular group of marking changes. The invariants  satisfy BCOV equations.

Generalizations

Higher order ramifications 

In case the  branchpoints are not simple, the definition is amended as follows (simple branchpoints correspond to k=2):

The first sum is over partitions  of  with non empty parts , and in the second sum, the prime means excluding all terms such that .

 is called the recursion kernel:

The base point * of the integral in the numerator can be chosen arbitrarily in a vicinity of the branchpoint, the invariants  will not depend on it.

Topological recursion invariants and intersection numbers 

The invariants  can be written in terms of intersection numbers of tautological classes

(*) 
where the sum is over dual graphs of stable nodal Riemann surfaces of total arithmetic genus , and  smooth labeled marked points , and equipped with a map .
 is the Chern class of the cotangent line bundle  whose fiber is the cotangent plane at .
 is the th Mumford's kappa class.
The coefficients , , , are the Taylor expansion coefficients of  and  in the vicinity of branchpoints as follows:
in the vicinity of a branchpoint  (assumed simple), a local coordinate is . The Taylor expansion of  near branchpoints ,  defines the coefficients 
.

The Taylor expansion at , defines the 1-forms coefficients 

whose Taylor expansion near a branchpoint  is 
.

Write also the Taylor expansion of 
.
Equivalently, the coefficients  can be found from expansion coefficients of the Laplace transform, and the coefficients  are the expansion coefficients of the log of the Laplace transform
 .

For example, we have

The formula (*) generalizes ELSV formula as well as Mumford's formula and Mariño-Vafa formula.

Some applications in enumerative geometry

Mirzakhani's recursion 
M. Mirzakhani's recursion for hyperbolic volumes of moduli spaces is an instance of topological recursion.
For the choice of spectral curve 

the n-form  is the Laplace transform of the Weil-Petersson volume

where  is the moduli space of hyperbolic surfaces of genus g with n geodesic boundaries of respective lengths , and  is the Weil-Petersson volume form.

The topological recursion for the n-forms , is then equivalent to Mirzakhani's recursion.

Witten–Kontsevich intersection numbers 
For the choice of spectral curve 

the n-form  is

where  is the Witten-Kontsevich intersection number of Chern classes of cotangent line bundles in the compactified moduli space of Riemann surfaces of genus g with n smooth marked points.

Hurwitz numbers 
For the choice of spectral curve 

the n-form  is

where  is the connected simple Hurwitz number of genus g with ramification : the number of branch covers of the Riemann sphere by a genus g connected surface, with 2g-2+n simple ramification points, and one point with ramification profile given by the partition .

Gromov–Witten numbers and the BKMP conjecture 

Let  a toric Calabi–Yau 3-fold, with Kähler moduli .
Its mirror manifold is singular over a complex plane curve  given by a polynomial equation , whose coefficients are functions of the Kähler moduli.
For the choice of spectral curve 
 with  the fundamental second kind differential on , 

According to the BKMP conjecture, the n-form  is

where 

is the genus g Gromov–Witten number, representing the number of holomorphic maps of a surface of genus g into , with n boundaries mapped to a special  Lagrangian submanifold .  is the 2nd relative homology class of the surface's image, and  are homology classes (winding number) of the boundary images.

The BKMP conjecture has since then been proven.

Notes

References 

Topology
Algebraic geometry
Mathematical physics
String theory