Mathematical Sciences Classification System
18-XX - Category theory; homological algebra {For commutative rings see 13Dxx, for associative rings 16Exx, for groups 20Jxx, for topological groups and related structures 57Txx; see also 55Nxx and 55Uxx for algebraic topology}
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                            18-00General reference works (handbooks, dictionaries, bibliographies, etc.)
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                            18-01Instructional exposition (textbooks, tutorial papers, etc.)
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                            18-02Research exposition (monographs, survey articles)
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                            18-03Historical (must also be assigned at least one classification number from Section 01)
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                            18-04Explicit machine computation and programs (not the theory of computation or programming)
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                            18-06Proceedings, conferences, collections, etc.
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                            (+12)General theory of categories and functors
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                            (+23)Special categories
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                            (+5)Categories and theories
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                            (+41)Categories with structure
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                            (+15)Abelian categories
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                            (+12)Categories and geometry
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                            (+25)