20 Algebra and Number theory
21 Elementary and classical algebra. Theory of invariants
22 Linear algebra. Matrix theory. Matrices determinants
23 Group theory and Group representations. Semigroups (See also: 44)
24 Commutative algebra. Ring theory and algebras. Local rings. Commutative rings. Lie algebras. Hopf algebras. Jordan algebras
25.1 Algebraic structures. Partially ordered sets. Lattices. Relations. Boolean rings and Boolean algebras. Categorical algebras
25.2 Homological algebra. Category theory. Algebraic K-theory
26 Field and Galois theory of equations