Adjoint functors | Category theory | Homotopy theory

Cotriple homology

In algebra, given a category C with a cotriple, the n-th cotriple homology of an object X in C with coefficients in a functor E is the n-th homotopy group of the E of the augmented simplicial object induced from X by the cotriple. The term "homology" is because in the abelian case, by the Dold–Kan correspondence, the homotopy groups are the homology of the corresponding chain complex. Example: Let N be a left module over a ring R and let . Let F be the left adjoint of the forgetful functor from the category of rings to Set; i.e., free module functor. Then defines a cotriple and the n-th cotriple homology of is the n-th left derived functor of E evaluated at M; i.e., . Example (algebraic K-theory): Let us write GL for the functor . As before, defines a cotriple on the category of rings with F free ring functor and U forgetful. For a ring R, one has: where on the left is the n-th K-group of R. This example is an instance of . (Wikipedia).

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Related pages

Dold–Kan correspondence | Algebraic K-theory | Homotopy group