A short entry in the Stanford Encyclopaedia of Philosophy by Joan R. Moschovakis.
A very brief overview of the subject by Alex Sakharov from MathWorld.
Wikipedia (free encyclopedia) article.
Math page of Frank Waaldijk, containing articles and PhD thesis on foundations of constructive mathematics and intuitionistic topology. Also links to other mathematicians in this field.
Porgi is a Proof-Or-Refutation Generator for Intuitionistic propositional logic, implementated by Allen Stoughton. Given a sequent, Porgi either finds a minimally sized, normal natural deduction of the sequent, or it finds a "small", tree-based Kripke countermodel of the sequent. It is written in Standard ML.
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