The Synthetic Hilbert Additive Group Scheme

Abstract

We construct a lift of the degree filtration on the integer valued polynomials to (even MU-based) synthetic spectra. Namely, we construct a bialgebra in modules over the evenly filtered sphere spectrum which base-changes to the degree filtration on the integer valued polynomials. As a consequence, we may lift the Hilbert additive group scheme to a spectral group scheme over ${\mathbb{A}}^1/{\mathbb{G}}_m$. We study the cohomology of its deloopings, and show that one obtains a lift of the filtered circle, studied in [MRT22]. At the level of quasi-coherent sheaves, one obtains lifts synthetic lifts of the $\mathbb{Z}$-linear $\infty$-categories of $S_{\mathrm{fil}}^1$-representations. Our constructions crucially rely on the use of the even filtration of Hahn--Raksit--Wilson; it is linearity with respect to the even filtered sphere that powers the results of this work.

Final version, to appear in Selecta