# Equivariant cohomology

(Redirected from Borel construction)

In mathematics, equivariant 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 ${\displaystyle \Lambda }$ of the homotopy quotient ${\displaystyle EG\times _{G}X}$:

${\displaystyle H_{G}^{*}(X;\Lambda )=H^{*}(EG\times _{G}X;\Lambda ).}$

If ${\displaystyle G}$ is the trivial group, this is the ordinary cohomology ring of ${\displaystyle X}$, whereas if ${\displaystyle X}$ is contractible, it reduces to the cohomology ring of the classifying space ${\displaystyle BG}$ (that is, the group cohomology of ${\displaystyle G}$ when G is finite.) If G acts freely on X, then the canonical map ${\displaystyle EG\times _{G}X\to X/G}$ is a homotopy equivalence and so one gets: ${\displaystyle H_{G}^{*}(X;\Lambda )=H^{*}(X/G;\Lambda ).}$

If X is a manifold, G a compact Lie group and ${\displaystyle \Lambda }$ is the field of real numbers or the field of complex numbers (the most typical situation), then the above cohomology may be computed using so-called Cartan model (see equivariant differential forms.)

The construction should not be confused as a more naive cohomology of invariant differential forms: if G is a compact Lie group, then, by the averaging argument[citation needed], any form may be made invariant; thus, cohomology of invariant differential forms does not yield new information.

The Koszul duality is known to hold between equivariant cohomology and ordinary cohomology.

## 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 ${\displaystyle X}$ by its ${\displaystyle G}$-action) in which ${\displaystyle X}$ 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 EGBG 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−1x): moreover, this diagonal action is free since it is free on EG. So we define the homotopy quotient XG 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 EGBG. This bundle XXGBG is called the Borel fibration.

## An example of a homotopy quotient

The following example is Proposition 1 of [1].

Let X be a complex projective algebraic curve. We identify X as a topological space with the set of the complex points ${\displaystyle X(\mathbb {C} )}$, 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 ${\displaystyle BG}$ is 2-connected and X has real dimension 2. Fix some smooth G-bundle ${\displaystyle P_{\text{sm}}}$ on X. Then any principal G-bundle on ${\displaystyle X}$ is isomorphic to ${\displaystyle P_{\text{sm}}}$. In other words, the set ${\displaystyle \Omega }$ 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 ${\displaystyle P_{\text{sm}}}$ or equivalently the set of holomorphic connections on X (since connections are integrable for dimension reason). ${\displaystyle \Omega }$ is an infinite-dimensional complex affine space and is therefore contractible.

Let ${\displaystyle {\mathcal {G}}}$ be the group of all automorphisms of ${\displaystyle P_{\text{sm}}}$ (i.e., gauge group.) Then the homotopy quotient of ${\displaystyle \Omega }$ by ${\displaystyle {\mathcal {G}}}$ classifies complex-analytic (or equivalently algebraic) principal G-bundles on X; i.e., it is precisely the classifying space ${\displaystyle B{\mathcal {G}}}$ of the discrete group ${\displaystyle {\mathcal {G}}}$.

One can define the moduli stack of principal bundles ${\displaystyle \operatorname {Bun} _{G}(X)}$ as the quotient stack ${\displaystyle [\Omega /{\mathcal {G}}]}$ and then the homotopy quotient ${\displaystyle B{\mathcal {G}}}$ is, by definition, the homotopy type of ${\displaystyle \operatorname {Bun} _{G}(X)}$.

## Equivariant characteristic classes

Let E be an equivariant vector bundle on a G-manifold M. It gives rise to a vector bundle ${\displaystyle {\widetilde {E}}}$ on the homotopy quotient ${\displaystyle EG\times _{G}M}$ so that it pulls-back to the bundle ${\displaystyle EG\times E\to EG\times M}$. An equivariant characteristic class of E is then an ordinary characteristic class of ${\displaystyle {\widetilde {E}}}$, which is an element of the completion of the cohomology ring ${\displaystyle H^{*}(EG\times _{G}M)=H_{G}^{*}(M)}$. (In order to apply the 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 ${\displaystyle H^{2}(M;\mathbb {Z} ).}$[1] 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 ${\displaystyle H_{G}^{2}(M;\mathbb {Z} )}$.

## Localization theorem

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

## Notes

1. ^ using Čech cohomology and the isomorphism ${\displaystyle H^{1}(M;\mathbb {C} ^{*})\simeq H^{2}(M;\mathbb {Z} )}$ given by the exponential map.

## References

• Atiyah, Michael; Bott, Raoul (1984), "The moment map and equivariant cohomology", Topology, 23
• Michel Brion, "Equivariant cohomology and equivariant intersection theory" [2]
• Goresky, Mark; Kottwitz, Robert; MacPherson, Robert (1998), "Equivariant cohomology, Koszul duality, and the localization theorem", Inventiones Mathematicae, 131: 25–83, doi:10.1007/s002220050197
• Hsiang, Wu-Yi (1975). Cohomology Theory of Topological Transformation Groups. New York: Springer.
• Tu, Loring W. (March 2011). "What Is . . . Equivariant Cohomology?" (PDF). Notices of the American Mathematical Society. 58 (03): 423–426.