Logarithmic Prismatic Cohomology I

Abstract

We introduce a logarithmic variant of the notion of $\delta$-rings, which we call $\delta_{\log}$-rings, and use it to define a logarithmic version of the prismatic site introduced by Bhatt and Scholze. In particular, this enables us to construct the Breuil-Kisin cohomology in the semistable case.