Condensed Sets and the Solovay Model

Abstract

We exhibit a geometric morphism from the Grothendieck topos representing the Solovay model to the $\hat{I}º$-pyknotic sets of Barwick--Haine and Clausen--Scholze. We then use the properties of this morphism and automatic continuity in the Solovay model to prove Clausen--Scholze's resolution of the Whitehead problem for discrete condensed abelian groups. We also exhibit an analogous internal $Ext$ computation between locally compact abelian groups in the Solovay model.

43 pages; comments welcome! The earlier proof of proposition 3.9 contained an error, the claim has thus been weakened to lemma B.3