The Segal conjecture for large objects and semiadditive lifts

Abstract

We prove a projection formula for homotopy fixed points of trivial actions of finite $p$-groups. More precisely, for every bounded-below, $p$-complete spectrum $X$ whose mod-$p$ homology is $I$-complete, the canonical map \[ (\mathbb S_p)^{BG}\otimes X \overset{\simeq}{\rightarrow} X^{BG} \] is an equivalence for every finite $p$-group $G$, where $I$ is the augmentation ideal of the Steenrod algebra. Building on earlier joint work with Burklund, our proof combines the Adams--Gunawardena--Miller calculation with an adaptation of Carlsson's induction. As an application, using the work of Yuan, we construct height-one semiadditive lifts of such spectra and show that their cohomology of $p$-finite spaces depends only on the $1$-type.