base pushout
1Seifert–van Kampen theorem — In mathematics, the Seifert–van Kampen theorem of algebraic topology, sometimes just called van Kampen s theorem, expresses the structure of the fundamental group of a topological space X, in terms of the fundamental groups of two open, path… …
2Ext functor — In mathematics, the Ext functors of homological algebra are derived functors of Hom functors. They were first used in algebraic topology, but are common in many areas of mathematics. Definition and computation Let R be a ring and let mathrm{Mod}… …
