
 Mathematical Subject Classification 
  The MSC is broken down into over 5,000 two, three, and fivedigit classfications, each corresponding to a discipline of mathematics (e.g., 11 = Number theory; 11B = Sequences and sets; 11B05 = Density, gaps, topology). 
  The current classification system, 2000 Mathematics Subject Classification (MSC2000), is a revision of the 1991 Mathematics Subject Classification, which is the classification that has been used by MR and Zbl since the beginning of 1991. 
  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} 
 www.ams.org /msc (607 words) 
