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algebraic K-theory

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Cohomology

cohomology

Special and general types

Special notions

Variants

Extra structure

Operations

Theorems

Contents

Idea

Algebraic K-theory is about natural constructions of cohomology theories/spectra from algebraic data such as commutative rings, symmetric monoidal categories and various homotopy theoretic refinements of these.

The algebraic K-theory of a commutative ring RR was originally defined to be the Grothendieck group of its category of projective modules. Under the relation between modules and vector bundles, this is directly analogous to the basic definition of topological K-theory and hence the common term. (In fact when applied to the stack of vector bundles then algebraic K-theory subsumes topological K-theory and also differential K-theory, see below).

There are canonical maps K 0(R)Pic(R)K_0(R)\to Pic(R) from the 0th algebraic K-theory of a ring to its Picard group and K 1(R)GL 1(R)K_1(R)\to GL_1(R) from the first algebraic K-theory group of RR to its group of units which are given in components by the determinant functor. This fact is sometimes used to motivate algebraic K-theory as a “generalization of linear algebra” (see e.g. this MO discussion). This is also how the traditional regulator of a number field relates to Beilinson regulators of algebraic K-theory.

More generally, following the axiomatics of generalized (Eilenberg-Steenrod) cohomology any algebraic K-theory should be given by a sequence of functors K iK_i from some suitable class of categories of “algebraic nature” to abelian groups, satisfying some natural conditions. Moreover, following the Brown representability theorem these groups should arise as the homotopy groups of a spectrum, the algebraic K-theory spectrum. Classical constructions producing this by combinatorial means are known as the Quillen Q-construction defined on Quillen exact categories and more generally the Waldhausen S-construction defined on Waldhausen categories.

For more on the history of the subject see (Arlettaz 04, Grayson 13) and see at at Algebraic K-theory, a historical perspective.

There are two ways to think of the traditional algebraic K-theory of a commutative ring more conceptually: on the one hand this construction is the group completion of the direct sum symmetric monoidal-structure on the category of modules, on the other hand it is the group completion of the addition operation expressed by short exact sequences in that category. This leads to the two modern ways of expressing and viewing algebraic K-theory:

  1. monoidal. The core of a symmetric monoidal category or more generally of a symmetric monoidal (∞,1)-category has a universal completion to an abelian ∞-group/connective spectrum optained by universally adjoining inverses to the symmetric monoidal operation – the ∞-group completion. This yields the concept of algebraic K-theory of a symmetric monoidal category and more generally that of algebraic K-theory of a symmetric monoidal (∞,1)-category;

  2. exact/stable. Analogously, inverting the addition operation expressed by the exact sequences in an abelian category or more generally in a stable (∞,1)-category yields the algebraic K-theory of a stable (∞,1)-category. Explicit ways to express this are known as the Quillen Q-construction and the Waldhausen S-construction. This turns out to be a universal construction in the context of non-commutative motives.

Here the second construction may be understood as first splitting the exact sequences and then applying the first construction to the resulting direct sum monoidal structure. Typically the first construction here contains more information but is harder to compute, and vice versa (see also MO-discussion here and here).

Both of these constructions produce a spectrum (hence representing a generalized (Eilenberg-Steenrod) cohomology theory) – called the K-theory spectrum – and the algebraic K-theory groups are the homotopy groups of that spectrum.

The classical case of the algebraic K-theory of a commutative ring RR is a special case of this general concept of algebraic K-theory by either forming the symmetric monoidal category (Mod(R),)(Mod(R), \oplus) and applying the abelian ∞-group-completion to that, or else forming the stable (∞,1)-category of chain complexes of RR-modules and applyong the Waldhausen S-construction to that. In both cases the result is a spectrum whose degree-0 homotopy group is the ordinary algebraic K-theory of RR as given by the Grothendieck group and whose higher homotopy groups are its higher algebraic K-theory groups.

Constructions

Classical constructions

See at

Via stable (,1)(\infty,1)-categories

There is a universal characterization of the construction of the K-theory spectrum K(A)K(A) of a stable (,1)(\infty,1)-category AA:

there is an (,1)(\infty,1)-functor

U:(,1)StabCatN U : (\infty,1)StabCat \to N

to a stable (,1)(\infty,1)-category which is universal with the property that it respects colimits and exact sequences in a suitable way. Given any stable (,1)(\infty,1)-category AA, its (connective or non-connective, depending on details) algebraic K-theory spectrum is the hom-object

K(A)Hom(U(Sp),U(A)), K(A) \simeq Hom(U(Sp), U(A)) \,,

where SpSp denotes the stable (,1)(\infty,1)-category of compact spectra.

See (Blumberg-Gepner-Tabuada 10).

Via symmetric monoidal (,1)(\infty,1)-categories

See at

Properties

Relation to non-commutative topology and non-commutative motives

geometric contextuniversal additive bivariant (preserves split exact sequences)universal localizing bivariant (preserves all exact sequences in the middle)universal additive invariantuniversal localizing invariant
noncommutative algebraic geometrynoncommutative motives Mot addMot_{add}noncommutative motives Mot locMot_{loc}algebraic K-theorynon-connective algebraic K-theory
noncommutative topologyKK-theoryE-theoryoperator K-theory

Regulators (relation to ordinary cohomology)

See at Beilinson regulator.

Relation to topological K-theory

Red-shift conjecture

chromatic homotopy theory

chromatic levelcomplex oriented cohomology theoryE-∞ ring/A-∞ ringreal oriented cohomology theory
0ordinary cohomologyEilenberg-MacLane spectrum HH \mathbb{Z}HZR-theory
0th Morava K-theoryK(0)K(0)
1complex K-theorycomplex K-theory spectrum KUKUKR-theory
first Morava K-theoryK(1)K(1)
first Morava E-theoryE(1)E(1)
2elliptic cohomologyelliptic spectrum Ell EEll_E
second Morava K-theoryK(2)K(2)
second Morava E-theoryE(2)E(2)
algebraic K-theory of KUK(KU)K(KU)
3 …10K3 cohomologyK3 spectrum
nnnnth Morava K-theoryK(n)K(n)
nnth Morava E-theoryE(n)E(n)BPR-theory
n+1n+1algebraic K-theory applied to chrom. level nnK(E n)K(E_n) (red-shift conjecture)
\inftycomplex cobordism cohomologyMUMR-theory

Examples

On monoidal stacks

Algebraic K-theory is traditionally applied to single symmetric monoidal/stable (∞,1)-categories, but to the extent that it is functorial it may just as well be applied to (∞,1)-sheaves with values in these.

Notably, applied to the monoidal stack of vector bundles (with connection) on the site of smooth manifolds, the K-theory of a monoidal category-functor produces a sheaf of spectra which is a form of differential K-theory and whose geometric realization is the topological K-theory spectrum. For more on this see at differential cohomology hexagon – Differential K-theory.

Types of categories for which a theory of algebraic K-theory exist include notably the notions

Concrete examples of interest include for instance

geometric contextuniversal additive bivariant (preserves split exact sequences)universal localizing bivariant (preserves all exact sequences in the middle)universal additive invariantuniversal localizing invariant
noncommutative algebraic geometrynoncommutative motives Mot addMot_{add}noncommutative motives Mot locMot_{loc}algebraic K-theorynon-connective algebraic K-theory
noncommutative topologyKK-theoryE-theoryoperator K-theory

References

Introductions

Surveys with accounts of the historical development include

An introductory textbook account is in

Classical

Original articles include

  • R. W. Thomason, Thomas Trobaugh, Higher algebraic K-theory of schemes and of derived categories, The Grothendieck Festschrift, 1990, 247-435.

  • Daniel Quillen, Higher algebraic K-theory, in Higher K-theories, pp. 85–147, Proc. Seattle 1972, Lec. Notes Math. 341, Springer 1973. A reference for classical constructions is

For discussion of stable phenomena in algebraic K-theory, see section 4 of

  • Ralph Cohen, Stability phenomena in the topology of moduli spaces (pdf)

Discussion of the comparison map between algebraic and topological K-theory includes

  • Jonathan Rosenberg, Comparison between algebraic and topological K-theory for Banach algebras and C *C^\ast-algebras (pdf)

Via stable (,1)(\infty,1)-categories

The stable (∞,1)-category theory picture is discussed in

(in terms of noncommutative motives) and in

Via symmetric monoidal (,1)(\infty,1)-categories

The perspective of algebraic K-theory of a symmetric monoidal (∞,1)-category is developed in

K-theory stacks

The system of infinite loop spaces of the algebraic K-theory spectrum regarded as an ∞-stack on the Nisnevich site and the principal ∞-bundles over it is considered in

implementing a suggestion stated in

Examples

  • Alexey Ananyevskiy, On the algebraic KK-theory of some homogeneous varieties (pdf)

Revised on August 25, 2014 23:24:18 by Urs Schreiber (89.204.138.168)