# Contents

This entry will be (for now) both about graph homology and about graph cohomology, which were originally introduced by Maxim Kontsevich; once the material grows, it can be separated into two entries. Kontsevich produced few version of the graph complex, the basic one attached to the operad of 3-valent ribbon graphs. A graph complex can be produced as an output from the Feynman transform of a modular operad.

## Graph homology

Graph homology $H\mathcal{G}_{\bullet\bullet}$ is the cohomology of the graph complex $(\mathcal{G}_{\bullet\bullet}, \partial)$ which is the free $\mathbf{C}$-vector space generated by isomorphism classes of oriented ribbon graphs modulo relation $(\Gamma,-\sigma) = - (\Gamma, \sigma)$ where $\Gamma$ is a ribbon graph with orientation $\sigma$. The differential is given by

$\partial(\Gamma) := \sum_{e\in E(\Gamma)\backslash Loop(\Gamma)} \Gamma/e$

where the sum is over edges $e$ which are not loops and $\Gamma/e$ is obtained from $\Gamma$ by contraction at edge $e$ (cf. ribbon graph). The map $\partial$ is really a differential ($\partial^2 = 0$) because two contractions in different order produce a different orientation. There is a canonical bigrading on the graph complex, where $\mathcal{G}_{ij}$ is generated by those graphs which have $i$ vertices and $j$ edges; the differential has bidegree $(-1,-1)$; each $\mathcal{G}_{ij}$ is finite-dimensional, while the whole complex is infinte-dimensional. Graph splits into a direct sum of subcomplexes labelled by the Euler characteristics of the underlying graph. The structure of a graph complex reflects a structure in the Chevalley-Eilenberg complex of a certain Lie algebra; and the graph homology to the relative Lie homology of that Lie algebra as shown by Kontsevich.

## Properties

### $L_\infty$-algebra structure

The Graph complex carries the structure of a dg-Lie algebra (L-infinity algebra) which acts on the space of choices of formal deformation quantization of Poisson manifolds. Its degree-0 chain homology is the Lie algebra of the Grothendieck-Teichmüller group.

The homology in negative degree vanishes and that in positive degree is still unknown, but computer experiements show that at least the third cohomology contains nontrivial elements.

The degree-0 homology is also isomorphic, up to one “scaling class”, to the 0th cohomology of the derivations of the E2 operad.

## Applications

…moduli spaces

…deformation theory

…Rozansky-Witten theory

…Vassiliev invariants

…description of the classifying space $BOut(F_n)$ of the group of outer automorphisms of a free group with $n$ generators

Graph complex controls the universal $L_\infty$-deformations of the space of polyvector fields.

## Generalizations

There are generalizations for $d$-algebras (algebras over little disc operad in higher dimension). The cohomological graph complex is then the case for $d=2$. There is also a “directed” version. On the other hand, graph complex

## Literature

• Vasily Dolgushev, Christopher L. Rogers, Thomas Willwacher, Kontsevich’s graph complex, GRT, and the deformation complex of the sheaf of polyvector fields, arxiv/1211.4230

• Damien Calaque, Carlo A. Rossi, Lectures on Duflo isomorphisms in Lie algebra and complex geometry, European Math. Soc. 2011

• S. A. Merkulov, Graph complexes with loops and wheels, in (Manin’s Festschrift:) Algebra, Arithmetic, and Geometry, Progress in Mathematics 270 (2009) 311-354, doi, pdf

The following survey has discussion of context between the graph complex and Batalin-Vilkovisky formalism:

• Jian Qiu, Maxim Zabzine, Introduction to graded geometry, Batalin-Vilkovisky formalism and their applications, arxiv/1105.2680
• Jian Qiu, Maxim Zabzine, Knot weight systems from graded symplectic geometry, arxiv/1110.5234
Revised on August 21, 2013 09:52:32 by Urs Schreiber (24.131.18.91)