# Equivariant cohomology

In mathematics, equivariant cohomology (or Borel cohomology) is a cohomology theory from algebraic topology which applies to topological spaces with a group action. It can be viewed as a common generalization of group cohomology and an ordinary cohomology theory. Specifically, the equivariant cohomology ring of a space $X$ with action of a topological group $G$ is defined as the ordinary cohomology ring with coefficient ring $\Lambda$ of the homotopy quotient $EG\times _{G}X$:
    $H_{G}^{*}(X;\Lambda )=H^{*}(EG\times _{G}X;\Lambda ).$
If $G$ is the trivial group, this is the ordinary cohomology ring of $X$, whereas if $X$ is contractible, it reduces to the cohomology ring of the classifying space $BG$ (that is, the group cohomology of $G$ when G is finite.) If G acts freely on X, then the canonical map $EG\times _{G}X\to X/G$ is a homotopy equivalence and so one gets: $H_{G}^{*}(X;\Lambda )=H^{*}(X/G;\Lambda ).$

## Definitions

It is also possible to define the equivariant cohomology $H_{G}^{*}(X;A)$ of $X$ with coefficients in a $G$-module A; these are abelian groups. This construction is the analogue of cohomology with local coefficients.
If X is a manifold, G a compact Lie group and $\Lambda$ is the field of real numbers or the field of complex numbers (the most typical situation), then the above cohomology may be computed using the so-called Cartan model (see equivariant differential forms.)
The construction should not be confused with other cohomology theories, such as Bredon cohomology or the cohomology of invariant differential forms: if G is a compact Lie group, then, by the averaging argument, any form may be made invariant; thus, cohomology of invariant differential forms does not yield new information.
Koszul duality is known to hold between equivariant cohomology and ordinary cohomology.

### Relation with groupoid cohomology

For a Lie groupoid ${\mathfrak {X}}=[X_{1}\rightrightarrows X_{0}]$ equivariant cohomology of a smooth manifold is a special example of the groupoid cohomology of a Lie groupoid. This is because given a $G$-space $X$ for a compact Lie group $G$, there is an associated groupoid
   ${\mathfrak {X}}_{G}=[G\times X\rightrightarrows X]$
whose equivariant cohomology groups can be computed using the Cartan complex $\Omega _{G}^{\bullet }(X)$ which is the totalization of the de-Rham double complex of the groupoid. The terms in the Cartan complex are
   $\Omega _{G}^{n}(X)=\bigoplus _{2k+i=n}({\text{Sym}}^{k}({\mathfrak {g}}^{\vee })\otimes \Omega ^{i}(X))^{G}$
where ${\text{Sym}}^{\bullet }({\mathfrak {g}}^{\vee })$ is the symmetric algebra of the dual Lie algebra from the Lie group $G$, and $(-)^{G}$ corresponds to the $G$-invariant forms. This is a particularly useful tool for computing the cohomology of $BG$ for a compact Lie group $G$ since this can be computed as the cohomology of
   $[G\rightrightarrows *]$
where the action is trivial on a point. Then,
   $H_{dR}^{*}(BG)=\bigoplus _{k\geq 0}{\text{Sym}}^{2k}({\mathfrak {g}}^{\vee })^{G}$
For example,
   ${\begin{aligned}H_{dR}^{*}(BU(1))&=\bigoplus _{k=0}{\text{Sym}}^{2k}(\mathbb {R} ^{\vee })\\&\cong \mathbb {R} [t]\\&{\text{ where }}\deg(t)=2\end{aligned}}$
since the $U(1)$-action on the dual Lie algebra is trivial.

## Homotopy quotient

The homotopy quotient, also called homotopy orbit space or Borel construction, is a “homotopically correct” version of the orbit space (the quotient of $X$ by its $G$-action) in which $X$ is first replaced by a larger but homotopy equivalent space so that the action is guaranteed to be free.
To this end, construct the universal bundle EG → BG for G and recall that EG admits a free G-action. Then the product EG × X —which is homotopy equivalent to X since EG is contractible—admits a “diagonal” G-action defined by (e,x).g = (eg,g$^{−1}$x): moreover, this diagonal action is free since it is free on EG. So we define the homotopy quotient X$_G$ to be the orbit space (EG × X)/G of this free G-action.
In other words, the homotopy quotient is the associated X-bundle over BG obtained from the action of G on a space X and the principal bundle EG → BG. This bundle X → X$_G$ → BG is called the Borel fibration.

## An example of a homotopy quotient

The following example is Proposition 1 of .
Let X be a complex projective algebraic curve. We identify X as a topological space with the set of the complex points $X(\mathbb {C} )$, which is a compact Riemann surface. Let G be a complex simply connected semisimple Lie group. Then any principal G-bundle on X is isomorphic to a trivial bundle, since the classifying space $BG$ is 2-connected and X has real dimension 2. Fix some smooth G-bundle $P_{\text{sm}}$ on X. Then any principal G-bundle on $X$ is isomorphic to $P_{\text{sm}}$. In other words, the set $\Omega$ of all isomorphism classes of pairs consisting of a principal G-bundle on X and a complex-analytic structure on it can be identified with the set of complex-analytic structures on $P_{\text{sm}}$ or equivalently the set of holomorphic connections on X (since connections are integrable for dimension reason). $\Omega$ is an infinite-dimensional complex affine space and is therefore contractible.
Let ${\mathcal {G}}$ be the group of all automorphisms of $P_{\text{sm}}$ (i.e., gauge group.) Then the homotopy quotient of $\Omega$ by ${\mathcal {G}}$ classifies complex-analytic (or equivalently algebraic) principal G-bundles on X; i.e., it is precisely the classifying space $B{\mathcal {G}}$ of the discrete group ${\mathcal {G}}$.
One can define the moduli stack of principal bundles $\operatorname {Bun} _{G}(X)$ as the quotient stack $[\Omega /{\mathcal {G}}]$ and then the homotopy quotient $B{\mathcal {G}}$ is, by definition, the homotopy type of $\operatorname {Bun} _{G}(X)$.

## Equivariant characteristic classes

Let E be an equivariant vector bundle on a G-manifold M. It gives rise to a vector bundle ${\widetilde {E}}$ on the homotopy quotient $EG\times _{G}M$ so that it pulls-back to the bundle ${\widetilde {E}}=EG\times E$ over $EG\times M$. An equivariant characteristic class of E is then an ordinary characteristic class of ${\widetilde {E}}$, which is an element of the completion of the cohomology ring $H^{*}(EG\times _{G}M)=H_{G}^{*}(M)$. (In order to apply Chern–Weil theory, one uses a finite-dimensional approximation of EG.)
Alternatively, one can first define an equivariant Chern class and then define other characteristic classes as invariant polynomials of Chern classes as in the ordinary case; for example, the equivariant Todd class of an equivariant line bundle is the Todd function evaluated at the equivariant first Chern class of the bundle. (An equivariant Todd class of a line bundle is a power series (not a polynomial as in the non-equivariant case) in the equivariant first Chern class; hence, it belongs to the completion of the equivariant cohomology ring.)
In the non-equivariant case, the first Chern class can be viewed as a bijection between the set of all isomorphism classes of complex line bundles on a manifold M and $H^{2}(M;\mathbb {Z} ).$ In the equivariant case, this translates to: the equivariant first Chern gives a bijection between the set of all isomorphism classes of equivariant complex line bundles and $H_{G}^{2}(M;\mathbb {Z} )$.

## Localization theorem

The localization theorem is one of the most powerful tools in equivariant cohomology.