> 39-XX Difference and functional equations > 39BXX Functional equations

39B05 General	39B12 Iteration theory, iterative and composite equations	39B22 Equations for real functions
39B32 Equations for complex functions	39B42 Matrix and operator equations	39B52 Equations for functions with more general domains and/or ranges
39B55 Orthogonal additivity and other conditional equations	39B62 Functional inequalities, including subadditivity, convexity, etc.	39B72 Systems of functional equations and inequalities
39B82 Stability, separation, extension, and related topics	39B99 None of the above, but in this section

39BXX Functional equations