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 $F(B{G}_{+},X{)}^G$.)

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

See also

• Segal conjecture

References

• Mathew, Akhil; Meier, Lennart (2015). "Affineness and chromatic homotopy theory". Journal of Topology 8 (2): 476–528. doi:10.1112/jtopol/jtv005. 
• 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 

External links

• "Homology of homotopy fixed point spectra". MathOverflow. June 30, 2012. https://mathoverflow.net/q/101011.