> 39-XX Difference and functional equations

39-00 General reference works (handbooks, dictionaries, bibliographies, etc.)	39-01 Instructional expositions (textbooks, tutorial papers, etc.)	39-02 Research expositions (monographs, survey articles)
39-03 Historical (must also be assigned at least one classification number from section 01)	39-04 Explicit machine computation and programs (not the theory of computation or programming)	39-06 Proceedings, conferences, collections, etc.
39-99 Difference and functional equations (not classified at a more specific level)	39AXX Difference equations	39BXX Functional equations

39-XX Difference and functional equations