## Posts tagged ‘baer sum’

### Extensions and homological algebra

If you ever followed a course in algebra, you may have heard about *extensions* of “things”. Most “things” fit in what is called a *category*, that is, an abstract structure remembering how to compose morphisms between these “things”, and sometimes what is the sum of such “things”, the kernel of a morphism: we are interested in the structure of abelian category, which is the framework of vector spaces, modules over a ring, sheaves of modules… An extension of A by B is an exact sequence

where is the kernel, and is the quotient (cokernel). The *group of extensions * is the set of isomorphism classes of such exact sequences. (more…)

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