a fibration sequence (i.e. a homotopy exact sequence) the abelian group operation
is such that
The group is called the K-group of or the Grothendieck group of . See in particular the latter entry for more details.
The “K” is chosen by Grothendieck for the German word Klasse for “class”. The K-group of is the group of equivalence classes of : it is a group due to the existence of a notion of exact sequences in .
K-theory starts with the study of these K-groups and their higher analogues. Sometimes the K-groups themselves are called “K-theory”. One would say for instance: “ is the K-theory of .”
In nice cases this is the degree 0 part of a non-connective spectrum which is then the K-theory spectrum of . This is also called the Waldhausen K-theory of .
Much of the literature on K-theory discusses constructions that model the above abstract setup in terms of model categories, or just their homotopy categories, often of the derived catgeories type and then often expressed in terms of the abelian category or more generally Quillen exact category from which the derived category is derived.
Only a subset of the structure on a model category is necessary in order to conveniently extract the K-groups of the presented stable (∞,1)-category. For that reason the axioms of a Waldhausen category have been devised to provide just the necessary convenient prerequisites to compute the K-groups of the (∞,1)-category presented by the underlying homotopical category.
the corresponding K-group is degree 0 topological K-theory. This was the original of the notion and the term K-theory.
For an (∞,1)-category and , write for the full sub--category on on those objects for which
Let be a stable (∞,1)-category. Then its Waldhausen K-theory
This construction is also conjectured in the last section of Toen-Vezzosi’s A remark on K-theory .
It should be true that with this definition we have an isomorphism of groups
This Waldhausen/hocolim-construction gives the connective K-theory, taking values in connective spectra. The universal completion to functor that sends homotopy cofibers of stable (infinity,1)-categories to homotopy cofibers of spectra is the corresponding unconnective -functor.
There is a universal characterization of the construction of the -theory spectrum of a stable -category :
there is an -functor
to a stable -category which is universal with the property that it respects colimits and exact sequences in a suitable way. Given any stable -category , its (connective or non-connective, depending on details) algebraic K-theory spectrum is the hom-object
It was in
that it was proven that the the Waldhausen S-construction of the K-theory spectrum depends precisely on the simplicial localization of the Waldhausen category, i.e. of the (∞,1)-category that it presents.
In view of this remark 11.4 in
interprets the construction of the K-theory spectrum as a natural operation of stable (∞,1)-categories, as described above.
The universal property of the -categorical definition is studied in
The standard constructions of K-theory spectra from Quillen exact categories are discussed in detail in chapter 1 of
A useful introduction to the definition and computation of K-groups (with a little on K-spectra) is