31-00 General reference works (handbooks, dictionaries, bibliographies, etc.)
31-01 Instructional exposition (textbooks, tutorial papers, etc.)
31-02 Research exposition (monographs, survey articles)
31-03 Historical (must also be assigned at least one classification number from Section 01)
31-04 Explicit machine computation and programs (not the theory of computation or programming)
31-06 Proceedings, conferences, collections, etc.
31Axx Two-dimensional theory
31Bxx Higher-dimensional theory
31Cxx Other generalizations
31D05 Axiomatic potential theory