Semi-purity for cycles with modulus

TL;DR

This paper establishes a purity property for a modulus-invariant replacement of a motivic object, leading to a homotopy invariance result crucial for the development of motives with modulus.

Contribution

It proves a purity property for the $(P^1, \infty)$-invariant replacement in the modulus setting, extending Voevodsky's homotopy invariance theorem.

Findings

Proves purity for modulus pairs involving smooth projective schemes.

Establishes homotopy invariance of cohomology for modulus sheaves.

Supports the construction of a homotopy t-structure in motives with modulus.

Abstract

In this paper, we prove a form of purity property for the $(\mathbb{P}^1, \infty)$-invariant replacement $h_0^{\overline{\square}}(\mathfrak{X})$ of the Yoneda object $\mathbb{Z}_{\rm tr} (\mathfrak{X})$ for a modulus pair $\mathfrak{X}=(\overline{X}, X_\infty)$ over a field $k$, consisting of a smooth projective $k$-scheme and an effective Cartier divisor on it. As application, we prove the analogue in the modulus setting of Voevodsky's fundamental theorem on the homotopy invariance of the cohomology of homotopy invariant sheaves with transfers, based on a main result of "Purity of reciprocity sheaves" arXiv:1704.02442. This plays an essential role in the development of the theory of motives with modulus, and among other things implies the existence of a homotopy $t$-structure on the category $\mathbf{MDM}^{\rm eff}(k)$ of Kahn-Saito-Yamazaki.