Derived Functor SemiTor
A complex C• over an exact category  A is called exact if it is composed of exact triples Zi → Ci → Zi+1 in A. A complex over A is called acyclic if it is homotopy equivalent to an exact complex (or equivalently, if it is a direct summand of an exact complex). Acyclic complexes form a thick subcategory Acycl(A) of the homotopy category Hot(A) of complexes over A. All acyclic complexes over A are exact if and only if A contains images of idempotent endomorphisms .
KeywordsFull Subcategory Coderived Category Abelian Category Tensor Category Total Complex
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