Dependent Sums and Dependent Products in Bishop’s Set Theory

According to the standard, non type-theoretic accounts of Bishop’s constructivism (BISH), dependent functions are not necessary to BISH. Dependent functions though, are explicitly used by Bishop in his definition of the intersection of a family of subsets, and they are necessary to the definition of arbitrary products. In this paper we present the basic notions and principles of CSFT, a semi-formal constructive theory of sets and functions intended to be a minimal, adequate and faithful, in Feferman’s sense, semi-formalisation of Bishop’s set theory (BST). We define the notions of dependent sum (or exterior union) and dependent product of set-indexed families of sets within CSFT, and we prove the distributivity of prod over sum i.e., the translation of the type-theoretic axiom of choice within CSFT. We also define the notions of dependent sum (or interior union) and dependent product of set-indexed families of subsets within CSFT. For these definitions we extend BST with the universe of sets #1 V_0 and the universe of functions #1 V_1.