English

A lift from group cohomology to spectra for trivial profinite actions

Algebraic Topology 2019-08-07 v1

Abstract

Let GG be a profinite group, XX a discrete GG-spectrum with trivial action, and XhGX^{hG} the continuous homotopy fixed points. For any NoGN \trianglelefteq_o G ("oo" for open), X=XNX = X^N is a G/NG/N-spectrum with trivial action. We construct a zigzag \text{colim}\,_N \,X^{hG/N} \buildrel\Phi\over\longrightarrow \text{colim}\,_N \,(X^{hN})^{hG/N} \buildrel\Psi\over\longleftarrow X^{hG}, where Ψ\Psi is a weak equivalence. When Φ\Phi is a weak equivalence, this zigzag gives an interesting model for XhGX^{hG} (for example, its Spanier-Whitehead dual is holimNF(XhG/N,S0)\text{holim}\,_N \,F(X^{hG/N}, S^0)). We prove that this happens in the following cases: (1) G<|G| < \infty; (2) XX is bounded above; (3) there exists {U}\{U\} cofinal in {N}\{N\}, such that for each UU, Hcs(U,π(X))=0H^s_c(U, \pi_\ast(X)) = 0, for s>0s > 0. Given (3), for each UU, there is a weak equivalence X\buildrelXhUX \buildrel\simeq\over\longrightarrow X^{hU} and XhGXhG/UX^{hG} \simeq X^{hG/U}. For case (3), we give a series of corollaries and examples. As one instance of a family of examples, if pp is a prime, K(np,p)K(n_p,p) the npn_pth Morava KK-theory K(np)K(n_p) at pp for some np1n_p \geq 1, and Zp\mathbb{Z}_p the pp-adic integers, then for each m2m \geq 2, (3) is satisfied when GpmZpG \leqslant \prod_{p \leq m} \mathbb{Z}_p is closed, X=p>m(HQK(np,p))X = \bigvee_{p > m} (H\mathbb{Q} \vee K(n_p,p)), and {U}:={NGNGoG}\{U\} := \{N_G \mid N_G \trianglelefteq_o G\}.

Keywords

Cite

@article{arxiv.1908.01898,
  title  = {A lift from group cohomology to spectra for trivial profinite actions},
  author = {Daniel G. Davis},
  journal= {arXiv preprint arXiv:1908.01898},
  year   = {2019}
}

Comments

18 pages; submitted for publication