Category Theory with Stratified Set Theory (1911.04704v2)

Abstract: This paper examines the category theory of stratified set theory (NF and KF). We work out the properties of the relevant categories of sets, and introduce a functorial analogue to Specker's T-operation. Such a development leads one to consider the appropriate notion of "elementary topos" for stratified set theories. In addition to considering the categorical properties of a generic model of NF set theory, we identify a stratified Yoneda Lemma and show NF encodes itself as a full internal subcategory. Finally, our desire to examine NF in the context of category theory motivates a more precise examination of strongly cantorian as an appropriate notion of smallness, replacing it with the notion of fibrewise strongly cantorian. In the absence of Choice, we introduce a new axiom (SCU) to NF, and examine some properties of NF + SCU.