18XX 


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} 

18Axx 

General theory of categories and functors 


18A05 
Definitions, generalizations 


18A10 
Graphs, diagram schemes, precategories [See especially 20L05] 


18A15 
Foundations, relations to logic and deductive systems [See also 03XX] 


18A20 
Epimorphisms, monomorphisms, special classes of morphisms, null morphisms 


18A22 
Special properties of functors (faithful, full, etc.) 


18A23 
Natural morphisms, dinatural morphisms 


18A25 
Functor categories, comma categories 


18A30 
Limits and colimits (products, sums, directed limits, pushouts, fiber products, equalizers, kernels, ends and coends, etc.) 


18A32 
Factorization of morphisms, substructures, quotient structures, congruences, amalgams 


18A35 
Categories admitting limits (complete categories), functors preserving limits, completions 


18A40 
Adjoint functors (universal constructions, reflective subcategories, Kan extensions, etc.) 


18A99 
None of the above, but in this section 