Some remarks on blueprints and ${\mathbb F}_1$-schemes

Abstract: Over the past two decades several different approaches to defining a geometry over ${\mathbb F}1$ have been proposed. In this paper, relying on To\"en and Vaqui\'e's formalism, we investigate a new category ${\mathsf{Sch}}{\widetilde{\mathsf B}}$ of schemes admitting a Zariski cover by affine schemes relative to the category of blueprints introduced by Lorscheid. A blueprint, that may be thought of as a pair consisting of a monoid $M$ and a relation on the semiring $M \otimes_{{\mathbb F}1} \mathbb N$, is a monoid object in a certain symmetric monoidal category $\mathsf B$, which is shown to be complete, cocomplete, and closed. We prove that every $\widetilde{\mathsf B}$-scheme $\Sigma$ can be associated, through adjunctions, with both a classical scheme $\Sigma{\mathbb Z}$ and a scheme $\underline{\Sigma}$ over ${\mathbb F}1$ in the sense of Deitmar, together with a natural transformation $\Lambda\colon \Sigma{\mathbb Z}\to \underline{\Sigma}\otimes_{{\mathbb F}_1} {\mathbb Z}$. Furthermore, as an application, we show that the category of "${\mathbb F}_1$-schemes" defined by A. Connes and C. Consani can be naturally merged with that of $\widetilde{\mathsf B}$-schemes to obtain a larger category, whose objects we call "${\mathbb F}_1$-schemes with relations".