# nLab G-spectrum

Contents

### Context

#### Stable Homotopy theory

stable homotopy theory

Introduction

# Contents

#### Representation theory

representation theory

geometric representation theory

# Contents

## Idea

For $G$ a compact Lie group (or more generally a compact topological group) the concept of $G$-spectrum (or $G$-equivariant spectrum) is the generalization of that of spectrum as one passes from stable homotopy theory to equivariant stable homotopy theory, or more generally, as $G$ is allow to vary, to global equivariant stable homotopy theory.

Where the ordinary concept of spectrum is given in terms of looping and delooping of ordinary topological spaces by ordinary spheres, a $G$-spectrum is instead given by looping and delooping of topological G-spaces with respect to representation spheres of $G$, namely one-point compactifications of linear $G$-representations, for all representations appearing in a chosen “G-universe”.

Such a G-universe is called complete if it contains every irreducible representation of $G$, and the spectra modeled on such a complete $G$-universe are the genuine $G$-spectra. At the other extreme, if the G-universe contains only the trivial representations, then the resulting spectra are the spectra with G-action, also called naive G-spectra for emphasis of the distinction to the previous case.

The genuine $G$-spectra are richer than spectra with G-action and have better homotopy-theoretic properties. In particular the genuine equivariant cohomology theories which they represent have suspension isomorphisms for suspension by all representation spheres and with respect to RO(G)-grading.

When $G$ is the trivial group, a $G$-spectrum is also known as a coordinate-free spectrum.

## Models

There are various equivalent ways to present genuine $G$-spectra.

### Indexed on all representations

Fix a G-universe. For $V$ any orthogonal representation in the universe, write $S^V$ for its representation sphere. For $V \hookrightarrow W$ a subrepresentation, write $W-V$ for the orthogonal complement representation.

A $G$-prespectrum $E$ is an assignment of a pointed G-space $E_V$ to each $G$-representation $V$ (in the given G-universe), equipped for each subrepresentation $V \hookrightarrow W$ with a pointed $G$-equivariant continuous function

$\sigma_{V,W} \;\colon\; S^{W-V} \wedge E_V \longrightarrow E_W$

such that

1. $\Sigma_{V,V} = id$;

2. for any $Z \hookrightarrow V \hookrightarrow W$ we have commuting diagrams

$\array{ S^{Z-W}\wedge S^{V-W} \wedge E_V &\stackrel{S^{Z-W}\wedge(\sigma_{V,W})}{\longrightarrow}& S^{Z-W}E_W \\ \downarrow && \downarrow^{\mathrlap{\Sigma_{Z,W}}} \\ S^{Z-V}E_V &\stackrel{\sigma_{V,Z}}{\longrightarrow}& E_Z }$

Write

$\tilde \sigma_{V,W} \;\colon\; E_V \longrightarrow \Omega^{W-V}E_W$

for the adjuncts of these structure maps.

A $G$-prespectrum is called (at least in (May 96, chapter XII))

• a $G$-$\Omega$-spectrum if all the $\sigma_{V,W}$ are weak homotopy equivalences;

• a $G$-spectrum if all the $\sigma_{V,W}$ are homeomorphisms.

### Via orthogonal spectra with $G$-action

While the definition of spectra indexed on all representations manifestly relates to the suspension isomorphism for smashing with representation spheres and shifting in RO(G)-grading, the information encoded in the objects in this definition has much redundancy. A “smaller” definition of genuine $G$-spectra is given by orthogonal spectra equipped with $G$-action (Mandell-May 04, Schwede 15).

### Via Mackey functors

For $G$ a finite group then genuine $G$-spectra are equivalent to Mackey functors on the category of finite G-sets.

### Via excisive functors

Characterization of $G$-spectra via excisive functors on G-spaces is in (Blumberg 05).

## Properties

### Relation to Borel-equivariant spectra

In the general context of (global) equivariant stable homotopy theory, Borel-equivariant spectra are those which are right induced from plain spectra, hence which are in the essential image of the right adjoint to the forgetful functor from equivariant spectra to plain spectra.

The equivariant version of the stable Whitehead theorem holds:

a map of $G$-spectra $f \colon E \longrightarrow F$ is a weak equivalence (e.g. an $RO(G)$-degree-wise weak homotopy equivalence of topological G-spaces in the model via indexing on all representations) precisely it if induces isomorphisms $\pi_\bullet(f) \colon \pi_\bullet(E) \longrightarrow \pi_{\bullet}(F)$ on all equivariant homotopy group Mackey functors.

## References

Good lecture notes are

The concept of genuine $G$-spectra is due to

• L. Gaunce Lewis, Jr., Peter May, M. Steinberger, and J. E. McClure, Equivariant stable homotopy theory, Lecture Notes in Mathematics, vol. 1213, Springer-Verlag, Berlin, 1986, With contributions by J. E. McClure. MR 866482 (88e:55002)

and in terms of orthogonal spectra due to

also (developed for the Arf-Kervaire invariant problem)

• Michael Hill, Michael Hopkins, Doug Ravenel, The Arf-Kervaire invariant problem in algebraic topology: introduction, Current developments in mathematics, 2009, Int. Press, Somerville, MA, 2010, pp. 23–57. MR 2757358

On the non-existence of elements of Kervaire invariant one, (arXiv:0908.3724)

The Arf-Kervaire problem in algebraic topology: sketch of the proof, Current developments in mathematics, 2010, Int. Press, Somerville, MA, 2011, pp. 1–43

Surveys and introductions include

Lecture notes include

• Peter May, Equivariant homotopy and cohomology theory CBMS Regional Conference Series in Mathematics, vol. 91, Published for the Conference Board of the Mathematical Sciences, Washington, DC, 1996. (pdf)

Relation to Mackey functors:

• Bert Guillou, Peter May, Models of $G$-spectra as presheaves of spectra, (arXiv:1110.3571)

Permutative $G$-categories in equivariant infinite loop space theory (arXiv:1207.3459)

• Denis Nardin, section 2.6 of Stability and distributivity over orbital ∞-categories, 2012 (pdf)

• Clark Barwick, Spectral Mackey functors and equivariant algebraic K-theory (I), Adv. Math., 304:646–727 (arXiv:1404.0108)

For more references see at equivariant stable homotopy theory and at Mackey functor

Characterization via excisive functors is in

In the case of a cyclic group of prime order, genuine $G$-spectra admit a simple model which amounts to specifying a spectrum $E$, a $G$-action on $E$, a genuine fixed point spectrum $E^G$, and a diagram $E_{hG} \to E^G \to E^{hG}$. See Example 3.29 in:

A perspective on the category of genuine G-spectra as a lax limit over those of “naive” G-spectra is given in

Last revised on October 24, 2018 at 14:31:41. See the history of this page for a list of all contributions to it.