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A topos is a category that behaves very much like the category of sets and possesses a good notion of localization. Related to topos are: sheaves, presheaves, descent, stacks, localization,...

4 votes

Topos theory reference suitable for undergraduates

Introduction to Higher Order Categorical Logic by Joachim Lambek and P. J. Scott. Try Google Books for this. The point of view is much more suitable for the functional programming aspects, even though …
Charles Matthews's user avatar
0 votes

Lattice of subcategories: subobject classifier in Cat

Special cases being the submonoids of a monoid, and suborders of a partial order. What would one expect to be a common generalisation of those two, that was of interest? All one really asks for is a c …
Charles Matthews's user avatar
4 votes

Stone Spaces, Locales, and Topoi for the (relative) beginner

The texts by Vickers and Johnstone are rather different, and certainly are different in intention. I was struck by a remark made to me by a leading computer scientist, to the effect that Stone Spaces …
Charles Matthews's user avatar
15 votes

Au revoir, law of excluded middle?

I don't know whether this will be helpful, but here goes. There used to be things called the "Laws of Thought", and they used to be equated (tendentiously) with sort-of axioms for rationality, when "a …
Charles Matthews's user avatar