Grothendieck-Witt theory of derived schemes

TL;DR

This paper constructs a motivic spectrum KO representing Grothendieck-Witt theory for derived schemes, establishing its properties, relations to other theories, and its non-$A^1$-invariance, with applications to classical and genuine Poincaré structures.

Contribution

It introduces a new motivic spectrum KO for Grothendieck-Witt theory, extending motivic homotopy theory to non-$A^1$-invariant contexts and classical Poincaré structures.

Findings

KO satisfies Nisnevich descent and blowup excision.

The fracture square relates GW-theory, L-theory, and K-theory.

KO is not Bott-periodic when 2 is not a unit.

Abstract

We construct a non-$\mathbb{A}^1$-invariant motivic ring spectrum $\mathrm{KO}$ over $\mathrm{Spec}(\mathbb{Z})$, whose associated cohomology theory on qcqs derived schemes is the Grothendieck-Witt theory of classical symmetric forms (as opposed to homotopy symmetric forms). In particular, we show that this theory satisfies Nisnevich descent, smooth blowup excision, a projective bundle formula, and is locally left Kan extended from smooth $\mathbb{Z}$-schemes up to Bass delooping. More generally, our construction produces $\mathrm{KO}$-modules representing localizing invariants of two different families of Poincar\'e structures on derived schemes, which we call "classical" and "genuine"; the latter Poincar\'e structures are defined for spectral schemes with involution, but the former only for derived schemes. We then establish basic properties of these motivic spectra. As in $\mathbb{A}^1$-homotopy theory, the fracture square of $\mathrm{KO}$ with respect to the Hopf element recovers the fundamental cartesian square relating GW-theory, L-theory, and K-theory. A new phenomenon when $2$ is not a unit is that $\mathrm{KO}$ is not Bott-periodic, and the left and right Bott periodizations of $\mathrm{KO}$ represent the Grothendieck-Witt theories of homotopy symmetric and homotopy quadratic forms, respectively. We also construct the expected metalinear $\mathrm{E}_\infty$-orientation of $\mathrm{KO}$. Finally, we show that the $\mathbb{A}^1$-localization of $\mathrm{KO}$ recovers the motivic spectrum recently constructed by Calm\`es, Harpaz, and Nardin.