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 $E=-\otimes _{R}N$. Let F be the left adjoint of the forgetful functor from the category of rings to Set; i.e., free module functor. Then $FU$ defines a cotriple and the n-th cotriple homology of $E(FU_{*}M)$ is the n-th left derived functor of E evaluated at M; i.e., $\operatorname {Tor} _{n}^{R}(M,N)$.

Example (algebraic K-theory): Let us write GL for the functor $R\mapsto \varinjlim _{n}GL_{n}(R)$. As before, $FU$ defines a cotriple on the category of rings with F free ring functor and U forgetful. For a ring R, one has:

$K_{n}(R)=\pi _{n-2}GL(FU_{*}R),\,n\geq 3$ 

where on the left is the n-th K-group of R. This example is an instance of nonabelian homological algebra.

References

• Weibel, Charles A. (1994). An introduction to homological algebra. Cambridge Studies in Advanced Mathematics. Vol. 38. Cambridge University Press. ISBN 978-0-521-55987-4. MR 1269324. OCLC 36131259.

Further reading

• Who Threw a Free Algebra in My Free Algebra?, a blog post.