Higher colimits, derived functors and homology

TL;DR

This paper develops a unified theory of higher colimits over categories of free presentations, linking various homology theories and K-theory through colimit constructions, and provides new formulas and sequences in algebraic homology.

Contribution

It introduces a comprehensive framework connecting homology functors, K-theory, and cyclic homology via higher colimits, offering new formulas and exact sequences.

Findings

Hochschild and cyclic homology can be expressed as higher colimits.

Connes' exact sequence is derived from a simple short exact sequence.

Third reduced K-functor is characterized as a colimit of the second reduced K-functor.

Abstract

A theory of higher colimits over categories of free presentations is developed. It is shown that different homology functors such as Hoshcshild and cyclic homology of algebras over a field of characteristic zero, simplicial derived functors, and group homology can be obtained as higher colimits of simply defined functors. Connes' exact sequence linking Hochschild and cyclic homology was obtained using this approach as a corollary of a simple short exact sequence. As an application of the developed theory it is shown that the third reduced $K$-functor can be defined as the colimit of the second reduced $K$-functor applied to the fibre square of a free presentation of an algebra. A Hopf-type formula for odd dimensional cyclic homology of an algebra over a field of characteristic zero is also proved.