geometric representation theory
representation, 2-representation, ∞-representation
Grothendieck group, lambda-ring, symmetric function, formal group
principal bundle, torsor, vector bundle, Atiyah Lie algebroid
Eilenberg-Moore category, algebra over an operad, actegory, crossed module
under construction
Equivariant stable homotopy theory over some topological group $G$ is the stable homotopy theory of G-spectra. This includes the naive G-spectra which constitute the actual stabilization of equivariant homotopy theory, but is more general, one speaks of genuine $G$-spectra. Notably a genuine $G$-spectrum has homotopy groups graded by the group of integers, but by the representation ring of $G$ (usually called RO(G)-grading).
The concept of cohomology in equivariant stable homotopy theory is equivariant cohomology:
cohomology in the presence of ∞-group $G$ ∞-action:
Borel equivariant cohomology | $\leftarrow$ | general (Bredon) equivariant cohomology | $\rightarrow$ | non-equivariant cohomology with homotopy fixed point coefficients |
---|---|---|---|---|
$\mathbf{H}(X_G, A)$ | trivial action on coefficients $A$ | $[X,A]^G$ | trivial action on domain space $X$ | $\mathbf{H}(X, A^G)$ |
The definition of G-spectrum is typically given in generalization of the definition of coordinate-free spectrum.
A G-universe in this context is (e.g. Greenlees-May, p. 10) an infinite dimensional real inner product space equipped with a linear $G$-action that is the direct sum of countably many copies of a given set of (finite dimensional? -DMR) representations of $G$, at least containing the trivial representation on $\mathbb{R}$ (so that $U$ contains at least a copy of $\mathbb{R}^\infty$).
Each such subspace of $U$ (representation contained in $U$? -DMR) is called an indexing space (RO(G)-grading). For $V \subset W$ indexing spaces, write $W-V$ for the orthogonal complement of $V$ in $W$. Write $S^V$ for the one-point compactification of $V$; and for $X$ any (pointed) topological space write $\Omega^V := [S^V,X]$ for the corresponding (based) sphere space.
A G-space in the following means a pointed topological space equipped with a continuous action of the topological group $G$ that fixes the base point. A morphism of $G$-spaces is a continuous map that fixes the basepoints and is $G$-equivariant.
A weak equivalence of $G$-spaces is a morphism that induces isomorphism on all $H$-fixed homotopy groups (…)
A $G$-spectrum $E$ (indexed on the chosen universe $U$) is
for each indexing space $V \subset U$ a $G$-space $E V$;
for each pair $V \subset W$ of indexing spaces a $G$-equivariant homeomorphism
A morphism $f : E \to E'$ of $G$-spectra is for each indexing space $V$ a morphism of $G$-spaces $f_V : E V \to E' V$, such that this makes the obvious diagrams commute.
This becomes a category with weak equivalences by setting:
a morphism $f$ of $G$-spectra is a weak equivalence of $G$-spectra if for every indexing space $V$ the component $f_V$ is a weak equivalence of $G$-spaces (as defined above).
This may be expressed directly in terms of the notion of homotopy group of a $G$-spectrum: this is …
… (Schwede 15)…
A Mackey functor with values in spectra (“spectral Mackey functor”) is an (∞,1)-functor on a suitable (∞,1)-category of correspondences $Corr_1^{eff}(\mathcal{C}) \hookrightarrow Corr_1(\mathcal{C})$ which sends coproducts to smash product. (This is similar to the concept of sheaf with transfer.)
For $G$ a finite group and $\mathcal{C}= G Set$ its category of permutation representations, then $S$ is a genuine $G$-equivariant spectrum (Guillou-May 11). So in this case the homotopy theory of spectral Mackey functors is a presentation for equivariant stable homotopy theory (Guillou-May 11, Barwick 14).
For $\mathcal{C}$ an abelian category this definition reduces (Barwick 14) Mackey functors as originally defined in (Dress 71).
The notion of cohomology relevant in equivariant stable homotopy theory is the flavor of equivariant cohomology (see there for details) called Bredon cohomology.
equivariant spectrum, equivariant sphere spectrum, equivariant suspension spectrum, equivariant homotopy group, equivariant stable homotopy category, tom Dieck splitting, slice spectral sequence
Original articles include
Introductions and surveys include
Gunnar Carlsson, A survey of equivariant stable homotopy theory,Topology, Vol 31, No. 1, pp. 1-27, 1992 (pdf)
Anna Marie Bohmann, Basic notions of equivariant stable homotopy theory (pdf)
John Greenlees, Peter May, Equivariant stable homotopy theory, in Ioan James (ed.), Handbook of Algebraic Topology , pp. 279-325. 1995. (pdf)
Brooke Shipley, An introduction to equivariant homotopy theory (pdf)
Lecture notes on G-spectra modeled as orthogonal spectra with $G$-actions are
A textbook account in terms of G-spectra modeled on a complete G-universe is in
and a more modern version taking into account the theory of symmetric monoidal categories of spectra is in
An alternative perspective on this is in
Generalization from equivariance under compact Lie groups to compact topological groups (Hausdorff) and in particular to profinite groups and pro-homotopy theory is in
The May recognition theorem for G-spaces and genuine G-spectra is discussed in
Characterization of G-spectra via excisive functors on G-spaces is in
The characterization of $G$-equivariant functors in terms of topological Mackey functors is discussed in example 3.4 (i) of
A comprehensive construction of equivariant stable homotopy theory in terms of Mackey functors is in the series
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)
A fully (∞,1)-category theoretic formulation is i