There's no 'universal' mathematics classification system. Several systems have come about over time
but I'm going to go with the agreed upon Mathematics Subject
Classification (MSC) scheme that's
used by the two major mathematical reviewing databases and used by many mathematics
journals.
mathematics classification
'common' classification areas
first level
second level
third level
General / Foundations
00: General
Philosophy of Mathematics
Mathematical Modeling
Recreational Mathematics
01: History & Biography
01-00: General reference works (handbooks, dictionaries, bibliographies, etc.)
Books