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Generalized geometric theories and set-generated classes

Published online by Cambridge University Press:  10 November 2014

PETER ACZEL
Affiliation:
Schools of Mathematics and Computer Science, University of Manchester, Manchester, M13 9PL, United Kingdom
HAJIME ISHIHARA
Affiliation:
School of Information Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan
TAKAKO NEMOTO
Affiliation:
School of Information Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan
YASUSHI SANGU
Affiliation:
School of Information Science, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923-1292, Japan

Abstract

We introduce infinitary propositional theories over a set and their models which are subsets of the set, and define a generalized geometric theory as an infinitary propositional theory of a special form. The main result is that the class of models of a generalized geometric theory is set-generated. Here, a class $\mathcal{X}$ of subsets of a set is set-generated if there exists a subset G of $\mathcal{X}$ such that for each α ∈ $\mathcal{X}$ , and finitely enumerable subset τ of α there exists a subset β ∈ G such that τ ⊆ β ⊆ α. We show the main result in the constructive Zermelo–Fraenkel set theory (CZF) with an additional axiom, called the set generation axiom which is derivable in CZF, both from the relativized dependent choice scheme and from a regular extension axiom. We give some applications of the main result to algebra, topology and formal topology.

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