# K-theory

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.[1]

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).

## Grothendieck completion

The Grothendieck completion is a necessary ingredient for constructing K-theory. Given an abelian monoid ${\displaystyle (A,+')}$ let ${\displaystyle \sim }$ be the relation on ${\displaystyle A^{2}}$ defined by

${\displaystyle (a_{1},a_{2})\sim (b_{1},b_{2})}$

if there exists a ${\displaystyle c\in A}$ such that ${\displaystyle a_{1}+'b_{2}+'c=a_{2}+'b_{1}+'c.}$ Then, the set ${\displaystyle G(A)=A^{2}/\sim }$ has the structure of a group ${\displaystyle (G(A),+)}$ where:

${\displaystyle [(a_{1},a_{2})]+[(b_{1},b_{2})]=[(a_{1}+'b_{1},a_{2}+'b_{2})]}$

Equivalence classes in this group should be thought of as formal differences of elements in the abelian monoid.

To get a better understanding of this group, consider some equivalence classes of the abelian monoid ${\displaystyle (A,+)}$. Here we will denote the identity element by ${\displaystyle 0}$. First, ${\displaystyle (0,0)\sim (n,n)}$ for any ${\displaystyle n\in A}$ since we can set ${\displaystyle c=0}$ and apply the equation from the equivalence relation to get ${\displaystyle n=n}$. This implies

${\displaystyle [(a,b)]+[(b,a)]=[(a+b,a+b)]=0}$

hence we have an additive inverse for each element in ${\displaystyle G(A)}$. This should give us the hint that we should be thinking of the equivalence classes ${\displaystyle [(a,b)]}$ as formal differences ${\displaystyle a-b}$. Another useful observation is the invariance of equivalence classes under scaling:

${\displaystyle (a,b)\sim (a+k,b+k)}$ for any ${\displaystyle k\in A}$

The Grothendieck completion can be viewed as a functor ${\displaystyle G:\mathbf {AbMon} \to \mathbf {AbGrp} }$, and it has the property that it is left adjoint to the corresponding forgetful functor ${\displaystyle U:\mathbf {AbGrp} \to \mathbf {AbMon} }$. That means that, given a morphism ${\displaystyle \phi :A\to U(B)}$ of an abelian monoid ${\displaystyle A}$ to the underlying abelian monoid of an abelian group ${\displaystyle B}$, there exists a unique abelian group morphism ${\displaystyle G(A)\to B}$.

An illustrative example to look at is the Grothendieck completion of ${\displaystyle \mathbb {N} }$. We can see that ${\displaystyle G((\mathbb {N} ,+))=(\mathbb {Z} ,+)}$. For any pair ${\displaystyle (a,b)}$ we can find a minimal representative ${\displaystyle (a',b')}$ by using the invariance under scaling. For example, we can see from the scaling invariance that

${\displaystyle (4,6)\sim (3,5)\sim (2,4)\sim (1,3)\sim (0,2)}$

In general, if we set ${\displaystyle k=\min\{a,b\}}$ then we find that

${\displaystyle (a,b)\sim (a-k,b-k)}$ which is of the form ${\displaystyle (c,0)}$ or ${\displaystyle (0,d)}$

This shows that we should think of the ${\displaystyle (a,0)}$ as positive integers and the ${\displaystyle (0,b)}$ as negative integers.

## Definitions

There are a number of basic definitions of K-theory: two coming from topology and two from algebraic geometry.

Given a compact Hausdorff space ${\displaystyle X}$ consider the set of isomorphism classes of finite-dimensional vector bundles over ${\displaystyle X}$, denoted ${\displaystyle {\text{Vect}}(X)}$ and let the isomorphism class of a vector bundle ${\displaystyle \pi :E\to X}$ be denoted ${\displaystyle [E]}$. Since isomorphism classes of vector bundles behave well with respect to direct sums, we can write these operations on isomorphism classes by

${\displaystyle [E]\oplus [E']=[E\oplus E']}$

It should be clear that ${\displaystyle ({\text{Vect}}(X),\oplus )}$ is an abelian monoid where the unit is given by the trivial vector bundle ${\displaystyle \mathbb {R} ^{0}\times X\to X}$. We can then apply the Grothendieck completion to get an abelian group from this abelian monoid. This is called the K-theory of ${\displaystyle X}$ and is denoted ${\displaystyle K^{0}(X)}$.

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 ${\displaystyle C^{0}(X;\mathbb {C} )}$ as projective modules. Then, these can be identified with idempotent matrices in some ring of matrices ${\displaystyle M_{n\times n}(C^{0}(X;\mathbb {C} ))}$. We can define equivalence classes of idempotent matrices and form an abelian monoid ${\displaystyle {\textbf {Idem}}(X)}$. Its Grothendieck completion is also called ${\displaystyle K^{0}(X)}$.

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 ${\displaystyle X}$. If we look at the isomorphism classes of coherent sheaves ${\displaystyle \operatorname {Coh} (X)}$ we can mod out by the relation ${\displaystyle [{\mathcal {E}}]=[{\mathcal {E}}']+[{\mathcal {E}}'']}$ if there is a short exact sequence

${\displaystyle 0\to {\mathcal {E}}'\to {\mathcal {E}}\to {\mathcal {E}}''\to 0.}$

This gives the Grothendieck-group ${\displaystyle K_{0}(X)}$ which is isomorphic to ${\displaystyle K^{0}(X)}$ if ${\displaystyle X}$ is smooth. The group ${\displaystyle K_{0}(X)}$ is special because there is also a ring structure: we define it as

${\displaystyle [{\mathcal {E}}]\cdot [{\mathcal {E}}']=\sum (-1)^{k}\left[\operatorname {Tor} _{k}^{{\mathcal {O}}_{X}}({\mathcal {E}},{\mathcal {E}}')\right].}$

Using the Grothendieck–Riemann–Roch theorem, we have that

${\displaystyle \operatorname {ch} :K_{0}(X)\otimes \mathbb {Q} \to A(X)\otimes \mathbb {Q} }$

is an isomorphism of rings. Hence we can use ${\displaystyle K_{0}(X)}$ for intersection theory.

## Early history

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".[2] 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.

If X is a smooth variety, the two groups are the same. If it is a smooth affine variety, then all extensions of locally free sheaves split, so the group has an alternative definition.

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.)

## Developments

The other historical origin of algebraic K-theory was the work of J. H. C. 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 corresponding constructions involving an auxiliary quadratic form received the general name L-theory. It is a major tool of surgery theory.

In string theory, the K-theory classification of Ramond–Ramond field strengths and the charges of stable D-branes was first proposed in 1997.[3]

## Examples

• The easiest example of the Grothendieck group is the Grothendieck group of a point ${\displaystyle {\text{Spec}}(\mathbb {F} )}$ for a field ${\displaystyle \mathbb {F} }$. 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 ${\displaystyle \mathbb {N} }$ corresponding to the dimension of the vector space. It is an easy exercise to show that the Grothendieck group is then ${\displaystyle \mathbb {Z} }$.
• One important property of the Grothendieck group of a Noetherian scheme ${\displaystyle X}$ is that ${\displaystyle K(X)=K(X_{\text{red}})}$.[4] Hence the Grothendieck group of any Artinian ${\displaystyle \mathbb {F} }$-algebra is ${\displaystyle \mathbb {Z} }$.
• Another important formula for the Grothendieck group is the projective bundle formula:[5] given a rank r vector bundle ${\displaystyle {\mathcal {E}}}$ over a Noetherian scheme ${\displaystyle X}$, the Grothendieck group of the projective bundle ${\displaystyle \mathbb {P} ({\mathcal {E}})=\operatorname {Proj} (\operatorname {Sym} ^{\bullet }({\mathcal {E}}^{\vee }))}$ is a free ${\displaystyle K(X)}$-module of rank r with basis ${\displaystyle 1,\xi ,\dots ,\xi ^{n-1}}$. This formula allows one to compute the Grothendieck group of ${\displaystyle \mathbb {P} _{\mathbb {F} }^{n}}$.

## Applications

### Virtual bundles

One useful application of the Grothendieck-group is to define virtual vector bundles. For example, if we have an embedding of smooth spaces ${\displaystyle Y\hookrightarrow X}$ then there is a short exact sequence

${\displaystyle 0\to \Omega _{Y}\to \Omega _{X}|_{Y}\to C_{Y/X}\to 0}$

where ${\displaystyle C_{Y/X}}$ is the conormal bundle of ${\displaystyle Y}$ in ${\displaystyle X}$. If we have a singular space ${\displaystyle Y}$ embedded into a smooth space ${\displaystyle X}$ we define the virtual conormal bundle as

${\displaystyle [\Omega _{X}|_{Y}]-[\Omega _{Y}]}$

Another useful application of virtual bundles is with the definition of a virtual tangent bundle of an intersection of spaces: Let ${\displaystyle Y_{1},Y_{2}\subset X}$ be projective subvarieties of a smooth projective variety. Then, we can define the virtual tangent bundle of their intersection ${\displaystyle Z=Y_{1}\cap Y_{2}}$ as

${\displaystyle [T_{Z}]^{vir}=[T_{Y_{1}}]|_{Z}+[T_{Y_{2}}]|_{Z}-[T_{X}]|_{Z}.}$

Kontsevich uses this construction in one of his papers.[6]

### Chern characters

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

${\displaystyle \operatorname {ch} (L)=\exp(c_{1}(L)):=\sum _{m=0}^{\infty }{\frac {c_{1}(L)^{m}}{m!}}.}$

More generally, if ${\displaystyle V=L_{1}\oplus \dots \oplus L_{n}}$ is a direct sum of line bundles, with first Chern classes ${\displaystyle x_{i}=c_{1}(L_{i}),}$ the Chern character is defined additively

${\displaystyle \operatorname {ch} (V)=e^{x_{1}}+\dots +e^{x_{n}}:=\sum _{m=0}^{\infty }{\frac {1}{m!}}(x_{1}^{m}+\dots +x_{n}^{m}).}$

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.

## Equivariant K-theory

The equivariant algebraic K-theory is an algebraic K-theory associated to the category ${\displaystyle \operatorname {Coh} ^{G}(X)}$ of equivariant coherent sheaves on an algebraic scheme ${\displaystyle X}$ with action of a linear algebraic group ${\displaystyle G}$, via Quillen's Q-construction; thus, by definition,

${\displaystyle K_{i}^{G}(X)=\pi _{i}(B^{+}\operatorname {Coh} ^{G}(X)).}$

In particular, ${\displaystyle K_{0}^{G}(C)}$ is the Grothendieck group of ${\displaystyle \operatorname {Coh} ^{G}(X)}$. The theory was developed by R. W. Thomason in 1980s.[7] Specifically, he proved equivariant analogs of fundamental theorems such as the localization theorem.