The spectral site revisited

TL;DR

This paper revisits the spectral construction through a site-theoretic lens, exploring geometric properties, classifying topoi, and applications to propositional algebras and logic theories.

Contribution

It provides a detailed site-theoretic account of spectral construction, new proofs of spectral adjunctions, and links spectra to classifying topoi of logical theories.

Findings

Spectral construction characterized via site theory.

New proofs of spectral adjunctions for models.

Examples include classifying topoi of logical theories.

Abstract

We give the site-theoretic account of the spectral construction as first introduced by Coste. We provide a detailed examination of the geometric properties of the spectrum, in particular what classes of topoi it produces when applied to the different classes of objects and maps in a geometry. We also give a new proof of the spectral adjunction for set-valued models and the classifying property of the structure sheaf. We also discuss the opposition between ``gros and petit" spectra and relation with some canonical classifying sites. We then describe the general case of a modelled topos. We also prove that the spectrum of a locally modelled topos is local over its base and deduce a new proof of the spectral adjunction for the general case. We finally give several examples from the world of propositional algebras, and in particular recover classifying topoi of first order theories in various fragment of logic as spectra of some suitably modelled topoi.