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Lecture notes by Maarten M. Fokkinga introducing some important notions from category theory, in particular adjunctions. Proofs are given in a calculational style, and the (few) examples are taken from algorithmics. The text is a long PostScript file.
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Section of the e-print arXiv dealing with category theory, including such topics as: enriched categories, topoi, abelian categories, monoidal categories, homological algebra.
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An archive of stories about category theorists.
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Web page for the category theory mailing list.
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Introductory article by John Baez.
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Jean-Pierre Marquis of the University of Montreal introduces the general mathematical theory of structures and systems of structures.
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In the "known maths" series.
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An implementation of concepts and constructions from category theory in the functional programming language Standard ML. Documentation and code.
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Maintained by M. Alsani.
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Maintained by Birant Ramazan. Address book, open problems, meetings, pictures, other resources.
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Notes by Ronald Brown.
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An illustrated guide book by Eugenia Cheng and Aaron Lauda (PS/PDF).
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Problems on model categories listed by Mark Hovey at Wesleyan University.
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Includes papers on category theory.
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By Michael Barr and Charles Wells, 1983. A revised and corrected version is now available free for downloading. Formats: DVI, PDF, PostScript.