In mathematics, K-theory is, roughly speaking, the study of a ring generated by vector bundles over a topological space or scheme. In algebraic topology, it is a cohomology theory known as topological K-theory. In algebra and algebraic geometry, it is referred to as algebraic K-theory. It is also a fundamental tool in the field of operator algebras. It can be seen as the study of certain kinds of invariants of large matrices.
K-theory involves the construction of families of K-functors that map from topological spaces or schemes to associated rings; these rings reflect some aspects of the structure of the original spaces or schemes. As with functors to groups in algebraic topology, the reason for this functorial mapping is that it is easier to compute some topological properties from the mapped rings than from the original spaces or schemes. Examples of results gleaned from the K-theory approach include the Grothendieck–Riemann–Roch theorem, Bott periodicity, the Atiyah-Singer index theorem, and the Adams operations.
In high energy physics, K-theory and in particular twisted K-theory have appeared in Type II string theory where it has been conjectured that they classify D-branes, Ramond–Ramond field strengths and also certain spinors on generalized complex manifolds. In condensed matter physics K-theory has been used to classify topological insulators, superconductors and stable Fermi surfaces. For more details, see K-theory (physics).
The Grothendieck completion is a necessary ingredient for constructing K-theory. Given an abelian monoid let be the relation on defined by
- if there exists a such that
Then, the set has the structure of a group where
Equivalence classes in this group should be thought of as formal differences of elements in the abelian group.
To get a better understanding of this group, let's look at some equivalence classes of the abelian monoid . Here we will denote the identity element by . First, notice that for any since we can set and apply the equation from the equivalence relation to get . Now, notice that this implies
hence we have an additive inverse for each element in . This should give us the hint that we should be thinking of the equivalence classes as formal differences . Another useful observation is the invariance of equivalence classes under scaling:
- for any
The Grothendieck completion can be viewed as a functor , and it has the nice property that it is left adjoint to the corresponding forgetful functor . That means, given a morphism of an abelian monoid to the underlying abelian monoid of an abelian group , there exists a unique abelian group morphism .
A nice illustrative example to look at is the Grothendieck completion of . We can see that . Notice that for any pair we can find a minimal representative by using the invariance under scaling. For example, we can see from the scaling invariance that
In general, if we set then we find that
- which is of the form or
This shows that we should think of the as positive integers and the as negative integers.
There are a number of basic definitions of K-theory: two coming from topology and two from algebraic geometry.
Given a compact Hausdorff space consider the set of isomorphism classes of finite-dimensional vector bundles over , denoted and let the isomorphism class of a vector bundle be denoted . Since isomorphism classes of vector bundles behave well with respect to direct sums, we can write these operations on isomorphism classes by
It should be clear that is an abelian monoid where the unit is given by the trivial vector bundle . We can then apply the Grothendieck completion to get an abelian group from this abelian monoid. This is called the K-theory of and is denoted .
We can use the Serre-Swan theorem and some algebra to get an alternative description of vector bundles over the ring of continuous complex valued functions as projective modules. Then, these can be identified with idempotent matrices in some ring of matrices . We can define equivalence classes of idempotent matrices and form an abelian monoid . Its Grothendieck completion is also called .
In algebraic geometry, the same construction can be applied to algebraic vector bundles over a smooth scheme. But, there is an alternative construction for any Noetherian scheme . If we look at the isomorphism classes of coherent sheaves we can mod out by the relation if there is a short exact sequence
This gives the Grothendieck-group which is isomorphic to if is smooth. The group is special because there is also a ring structure: we define it as
Using Grothendieck-Riemann-Roch we have that
is an isomorphism of rings. Hence we can use for intersection theory.
The subject can be said to begin with Alexander Grothendieck (1957), who used it to formulate his Grothendieck–Riemann–Roch theorem. It takes its name from the German Klasse, meaning "class". Grothendieck needed to work with coherent sheaves on an algebraic variety X. Rather than working directly with the sheaves, he defined a group using isomorphism classes of sheaves as generators of the group, subject to a relation that identifies any extension of two sheaves with their sum. The resulting group is called K(X) when only locally free sheaves are used, or G(X) when all are coherent sheaves. Either of these two constructions is referred to as the Grothendieck group; K(X) has cohomological behavior and G(X) has homological behavior.
In topology, by applying the same construction to vector bundles, Michael Atiyah and Friedrich Hirzebruch defined K(X) for a topological space X in 1959, and using the Bott periodicity theorem they made it the basis of an extraordinary cohomology theory. It played a major role in the second proof of the Index Theorem (circa 1962). Furthermore, this approach led to a noncommutative K-theory for C*-algebras.
Already in 1955, Jean-Pierre Serre had used the analogy of vector bundles with projective modules to formulate Serre's conjecture, which states that every finitely generated projective module over a polynomial ring is free; this assertion is correct, but was not settled until 20 years later. (Swan's theorem is another aspect of this analogy.)
The other historical origin of algebraic K-theory was the work of Whitehead and others on what later became known as Whitehead torsion.
There followed a period in which there were various partial definitions of higher K-theory functors. Finally, two useful and equivalent definitions were given by Daniel Quillen using homotopy theory in 1969 and 1972. A variant was also given by Friedhelm Waldhausen in order to study the algebraic K-theory of spaces, which is related to the study of pseudo-isotopies. Much modern research on higher K-theory is related to algebraic geometry and the study of motivic cohomology.
- The easiest example of the Grothendieck group is the Grothendieck group of a point for a field . Since a vector bundle over this space is just a finite dimensional vector space, which is a free object in the category of coherent sheaves, hence projective, the monoid of isomorphism classes is corresponding to the dimension of the vector space. It is an easy exercise to show that the Grothendieck group is then .
- One important property of the Grothendieck group of a Noetherian scheme is that . Hence the Grothendieck group of any Artinian -algebra is .
- Another important formula for the Grothendieck group is the projective bundle formula: given a rank r vector bundle over a Noetherian scheme , the Grothendieck group of the projective bundle is a free -module of rank r with basis . This formula allows one to compute the Grothendieck group of .
One useful application of the Grothendieck-group is to define virtual vector bundles. For example, if we have an embedding of smooth spaces then there is a short exact sequence
- where is the conormal bundle of in
If we have a singular space embedded into a smooth space we define the virtual conormal bundle as
Another useful application of virtual bundles is with the definition of a virtual tangent bundle of an intersection of spaces: Let be projective subvarieties of a smooth projective variety. Then, we can define the virtual tangent bundle of their intersection as
Kontsevich uses this construction in his paper 
Chern classes can be used to construct a homomorphism of rings from the topological K-theory of a space to (the completion of) its rational cohomology. For a line bundle L, the Chern character ch is defined by
More generally, if is a direct sum of line bundles, with first Chern classes the Chern character is defined additively
The Chern character is useful in part because it facilitates the computation of the Chern class of a tensor product. The Chern character is used in the Hirzebruch–Riemann–Roch theorem.
The equivariant algebraic K-theory is an algebraic K-theory associated to the category of equivariant coherent sheaves on an algebraic scheme with action of a linear algebraic group , via Quillen's Q-construction; thus, by definition,
In particular, is the Grothendieck group of . The theory was developed by R. W. Thomason in 1980s. Specifically, he proved equivariant analogs of fundamental theorems such as the localization theorem.
- Bott periodicity
- List of cohomology theories
- Algebraic K-theory
- Topological K-theory
- Operator K-theory
- Grothendieck–Riemann–Roch theorem
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