Classifying topoi and the axiom of infinity

Andreas Blass
1989 Algebra Universalis  
Let 6 e be an elementary topos. The axiom of infinity, asserting that 5 e has a natural numbers object, is shown to be necessary-sufficiency has long been known-for the existence of an object-classifying topos over 5 e.
doi:10.1007/bf01211840 fatcat:nbwueiqid5fihkge7di5iiihky