G-spectrum

In algebraic topology, a G-spectrum is a spectrum with an action of a (finite) group.

Let X be a spectrum with an action of a finite group G. The important notion is that of the homotopy fixed point set $X^{{hG}}$. There is always

$X^{G}\to X^{{hG}},$

a map from the fixed point spectrum to a homotopy fixed point spectrum (because, by definition, $X^{{hG}}$ is the mapping spectrum $F(BG_{+},X)^{G}$.)

Example: $\mathbb {Z} /2$ acts on the complex K-theory KU by taking the conjugate bundle of a complex vector bundle. Then $KU^{{h{\mathbb {Z}}/2}}=KO$, the real K-theory.

The cofiber of $X_{{hG}}\to X^{{hG}}$ is called the Tate spectrum of X.

G-Galois extension in the sense of Rognes

This notion is due to J. Rognes (Rognes 2008). Let A be an E_∞-ring with an action of a finite group G and B = A^hG its invariant subring. Then B → A (the map of B-algebras in E_∞-sense) is said to be a G-Galois extension if the natural map

$A\otimes _{B}A\to \prod _{{g\in G}}A$

(which generalizes $x\otimes y\mapsto (g(x)y)$ in the classical setup) is an equivalence. The extension is faithful if the Bousfield classes of A, B over B are equivalent.

Example: KO → KU is a ℤ./2-Galois extension.

See also

• Segal conjecture

References

• Mathew, Akhil; Meier, Lennart (2015). "Affineness and chromatic homotopy theory". arXiv:1311.0514.
• Rognes, John (2008), "Galois extensions of structured ring spectra. Stably dualizable groups", Memoirs of the American Mathematical Society, 192 (898), doi:10.1090/memo/0898, MR 2387923

External links

• http://mathoverflow.net/questions/101011/homology-of-homotopy-fixed-point-spectra