Wednesday, 7 November 2012

Category and homotopy theoretic methods in set theory

There is an interaction between category theory and set theory. In 1965, one year after Cohen's proof of the independence of the continuum hypothesis, Vopenka gave a proof using sheaf theory, see here.



This is nowadays systematized in the topos theoretic interpretation of set theory, for which you should look up MacLane/Moerdijk, as Dylan Wilson pointed out. The authors give a proof of the independence of the continuum hypothesis.



Marta Bunge has given a topos theoretic proof of the independence of the Suslin hypothesis from ZFC in: Marta Bunge, Topos Theory and Souslin's Hypothesis. J.Pure & Applied Algebra 4 (1974) 159-187.



As for the reformulation of Vopenka's principle: It is equivalent to the statement that a locally presentable category can not have a large full discrete subcategory. This, and more, is nicely explained in Adamek/Rosicky's Locally Presentable and Accessible Categories



On the other hand I know of no worked out connection between homotopy theory and set theory, just the indirect one via type theory mentioned by Andrej.

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