Solution to Lawvere's first problem: a Grothendieck topos that has proper class many quotient topoi
arXiv:2407.17105 · doi:10.1016/j.aim.2025.110751
Abstract
This paper solves the first of the open problems in topos theory posted by William Lawvere, concerning the existence of a Grothendieck topos that has proper class many quotient topoi. This paper concretely constructs such Grothendieck topoi, including the presheaf topos on the free monoid generated by countably infinitely many elements PSh(M_ω). Utilizing the combinatorics of the classifying topos of the theory of inhabited objects and with the help of a system of pairing functions, the problem is reduced to a theorem of Vopenka, Pultr, and Hedrlin, which states that any set admits a rigid relational structure.
arXiv commentv3: 19 pages, comments welcome