The independence of premise rule in intuitionistic set theories
Takako Nemoto, Michael Rathjen · arXiv (Cornell University) · 2019
Independence of premise principles play an important role in characterizing the modified realizability and the Dialectica interpretations. In this paper we show that a great many intuitionistic set theories are closed under the corresponding independence of premise rule for finite types over $\mathbb{N}$. It is also shown that the existence property (or existential definability property) holds for statements of the form $ eg A\to \exists x^σ F(x^σ)$, where the variable $x^σ$ ranges over a finite type $σ$. This applies in particular to Constructive Zermelo-Fraenkel Set Theory (CZF) and Intuitionistic Zermelo-Fraenkel Set Theory (IZF), two systems known not to have the general existence property. On the technical side, the paper uses the method of realizability with truth from [21] and [8] with the underlying partial combinatory algebra (pca) chosen among the total ones. A particular instance of the latter is provided by the substructure of the graph model formed by the semi computable subsets of $\mathbb{N}$, which has the advantage that it forms a set pca even in proof-theoretically weak set theories such as CZF.