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replaced http://mathoverflow.net/ with https://mathoverflow.net/

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edited Apr 13, 2017 at 12:58

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The category of groupoids as a categorification of the ring of rational numbers. See this MO question this MO question and this n-category cafe post.

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created Oct 26, 2010 at 11:28

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The category of groupoids as a categorification of the ring of rational numbers. See this MO question and this n-category cafe post.