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Keywords:

  • Axiom of choice;
  • axiom of extensionality;
  • weak set theory;
  • law of excluded middle

Abstract

A weak form of intuitionistic set theory WST lacking the axiom of extensionality is introduced. While WST is too weak to support the derivation of the law of excluded middle from the axiom of choice, we show that bee.ng up WST with moderate extensionality principles or quotient sets enables the derivation to go through. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)