| 16-xx | Associative rings and algebras {For the commutative case, see 13-xx} |
| 17-xx | Nonassociative rings and algebras |
| 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} |
| 19-xx | $K$-theory [See also 16E20, 18F25] |