nLab equivariant complex oriented cohomology theory

Idea

Equivariant complex oriented cohomology theory is the generalization of complex oriented cohomology theory to equivariant cohomology.

In generalization of topological K-theory as the prototypical example of a complex oriented cohomology theory, its generalization to equivariant K-theory is equivariantly complex oriented.

Examples

Equivariant complex K-theory

equivariant complex K-theory is an equivariant complex oriented cohomology theory (Greenlees 01, Section 10):

Proposition

(equivariant K-theory of projective G-space)

For $G$ an abelian compact Lie group, let

$$\underset{i}{\oplus} \, \mathbf{1}_{V_i} \;\; \in \;\; G Representations_{\mathbb{C}}^{fin}$$

be a finite-dimensional direct sum of complex 1-dimensional linear representations.

The $G$-equivariant K-theory ring $K_G(-)$ of the corresponding projective G-space $P(-)$ is the following quotient ring of the polynomial ring in one variable $L$ over the complex representation ring $R(G)$ of $G$:

(1)$$K_G \Big( P \big( \underset{i}{\oplus} \, \mathbf{1}_{V_i} \big) \Big) \;\; \simeq \;\; R(G) \big[ L \big] \big/ \underset{i}{\prod} \big( 1 - 1_{{}_{V_i}} L \big) \,,$$

where

• $L \,=\, \big[ \mathcal{L}_{ \underset{i}{\oplus} \mathbf{1}_{V_i} }\big]$ is the K-theory class of the tautological equivariant line bundle on the given projective G-space;

• $1_{{}_{V_i}} L \;=\; \big[ \mathbf{1}_{V_i} \boxtimes \mathcal{L}_{ \underset{i}{\oplus} \mathbf{1}_{V_i} } \big]$ is the class of its external tensor product of equivariant vector bundles with the given linear representation.

Corollary

(equivariant complex orientation of equivariant K-theory)

For $G$ an abelian compact Lie group and $\mathbf{1}_V \,\in\, G Representations_{\mathbb{C}}^{fin}$ a complex 1-dimensional linear representation, the corresponding representation sphere is the projective G-space $S^{\mathbf{1}_V} \,\simeq\, P\big( \mathbf{1}_V \oplus \mathbf{1} \big)$ (this Prop.) and so, by Prop. ,

$$\begin{aligned} \widetilde K_G \big( S^{\mathbf{1}_V} \big) & \simeq\, K_G \big( P( \mathbf{1}_V \oplus \mathbf{1} ) ; \, \underset{ \simeq \, pt }{ \underbrace{ P( \mathbf{1} ) } } \big) \\ & \simeq ker \Big( R(G)\big[L\big] \big/ (1 - 1_{{}_{V}} L) (1 - L) \longrightarrow \underset{ \simeq \, R(G) }{ \underbrace{ R(G)\big[L\big] \big/ (1 - L) } } \Big) \\ & \simeq (1 - L) \cdot R(G)\big[L\big] \big/ (1 - 1_{{}_{V}} L) (1 - L) \end{aligned}$$

is generated by the Bott element $(1 - L)$ over $P\big( \mathbf{1}_V \oplus \mathbf{1} \big)$. By the nature of the tautological equivariant line bundle, this Bott element is the restriction of that on infinite complex projective G-space $P\big(\mathcal{U}_G\big)$. The latter is thereby exhibited as an equivariant complex orientation in equivariant complex K-theory.

(Greenlees 01, p. 248 (24 of 39))

Equivariant complex cobordism

For an abelian compact Lie group $G$, equivariant complex cobordism theory $MU_G$ is an equivariant complex oriented cohomology theory (Greenlees 01, Sec. 13).

Much as in the non-equivariant case (see at universal complex orientation on MU), $MU_G$ is universal in that there is a bijection between equivariant complex orientations (in degree 2) on some cohomology theory $E_G$ and homotopy ring homomorphisms of $G$-spectra $MU_G \to E_G$ (Cole-Greenlees-Kriz 02, Theorem 1.2).

For the analogous statement on the equivariant Lazard ring see Greenlees 01a, Greenlees 01, Theorem 13.1, Cole-Greenlees-Kriz 02, Theorem 1.3.

Related concepts

• equivariant formal group

• projective G-space

• tautological equivariant line bundle

• real oriented cohomology theory, quaternionic oriented cohomology theory

References

• John Greenlees, Equivariant formal group laws and complex oriented cohomology theories, Homology Homotopy Appl. Volume 3, Number 2 (2001), 225-263 (euclid:hha/1139840255)

• Michael Cole, John Greenlees, Igor Kriz, Equivariant Formal Group Laws, Proceedings of the LMS, Volume 81, Issue 2 2000 (doi:10.1112/S0024611500012466)

• Michael Cole, John Greenlees, Igor Kriz, The universality of equivariant complex bordism, Math Z 239, 455–475 (2002) (doi:10.1007/s002090100315)

See also:

• Steven Costenoble, Peter May, Stefan Waner, Equivariant orientation theory, Homology Homotopy Appl.

  Volume 3, Number 2 (2001), 265-339 (euclid:hha/1139840256)