Classification of Clifford algebras


In abstract algebra, in particular in the theory of nondegenerate quadratic forms on vector spaces, the structures of finite-dimensional real and complex Clifford algebras for a nondegenerate quadratic form have been completely classified. In each case, the Clifford algebra is algebra isomorphic to a full matrix ring over R, C, or H, or to a direct sum of two copies of such an algebra, though not in a canonical way. Below it is shown that distinct Clifford algebras may be algebra-isomorphic, as is the case of Cl2,0 and Cl1,1, which are both isomorphic to the ring of two-by-two matrices over the real numbers.

Notation and conventions

The Clifford product is the manifest ring product for the Clifford algebra, and all algebra homomorphisms in this article are with respect to this ring product. Other products defined within Clifford algebras, such as the exterior product, are not used here. This article uses the sign convention for Clifford multiplication so that
for all vectors, where Q is the quadratic form on the vector space V. We will denote the algebra of matrices with entries in the division algebra K by Mn or. The direct sum of two such identical algebras will be denoted by, which is isomorphic to.

Bott periodicity

Clifford algebras exhibit a 2-fold periodicity over the complex numbers and an 8-fold periodicity over the real numbers, which is related to the same periodicities for homotopy groups of the stable unitary group and stable orthogonal group, and is called Bott periodicity. The connection is explained by the geometric model of loop spaces approach to Bott periodicity: their 2-fold/8-fold periodic embeddings of the classical groups in each other, and their successive quotients are symmetric spaces which are homotopy equivalent to the loop spaces of the unitary/orthogonal group.

Complex case

The complex case is particularly simple: every nondegenerate quadratic form on a complex vector space is equivalent to the standard diagonal form
where, so there is essentially only one Clifford algebra in each dimension. This is because the complex numbers include by which and so positive or negative terms are equivalent. We will denote the Clifford algebra on Cn with the standard quadratic form by Cln.
There are two separate cases to consider, according to whether n is even or odd. When n is even the algebra Cln is central simple and so by the Artin–Wedderburn theorem is isomorphic to a matrix algebra over C. When n is odd, the center includes not only the scalars but the pseudoscalars as well. We can always find a normalized pseudoscalar ω such that. Define the operators
These two operators form a complete set of orthogonal idempotents, and since they are central they give a decomposition of Cln into a direct sum of two algebras
where
The algebras are just the positive and negative eigenspaces of ω and the P± are just the projection operators. Since ω is odd, these algebras are mixed by α :
and therefore isomorphic. These two isomorphic algebras are each central simple and so, again, isomorphic to a matrix algebra over C. The sizes of the matrices can be determined from the fact that the dimension of Cln is 2n. What we have then is the following table:
nCln
2mM
2m+1M ⊕ M

The even subalgebra of Cln is isomorphic to Cln−1. When n is even, the even subalgebra can be identified with the block diagonal matrices. When n is odd, the even subalgebra are those elements of for which the two factors are identical. Picking either piece then gives an isomorphism with.

Real case

The real case is significantly more complicated, exhibiting a periodicity of 8 rather than 2, and there is a 2-parameter family of Clifford algebras.

Classification of quadratic forms

Firstly, there are non-isomorphic quadratic forms of a given degree, classified by signature.
Every nondegenerate quadratic form on a real vector space is equivalent to the standard diagonal form:
where is the dimension of the vector space. The pair of integers is called the signature of the quadratic form. The real vector space with this quadratic form is often denoted Rp,q. The Clifford algebra on Rp,q is denoted Clp,q.
A standard orthonormal basis for Rp,q consists of mutually orthogonal vectors, p of which have norm +1 and q of which have norm −1.

Unit pseudoscalar

The unit pseudoscalar in Clp,q is defined as
This is both a Coxeter element of sorts and a longest element of a Coxeter group in the Bruhat order; this is an analogy. It corresponds to and generalizes a volume form, and lifts reflection through the origin.
To compute the square, one can either reverse the order of the second group, yielding, or apply a perfect shuffle,
yielding. These both have sign, which is 4-periodic, and combined with, this shows that the square of ω is given by
Note that, unlike the complex case, it is not always possible to find a pseudoscalar that squares to +1.

Center

If n is even, the algebra Clp,q is central simple and so isomorphic to a matrix algebra over R or H by the Artin–Wedderburn theorem.
If n is odd then the algebra is no longer central simple but rather has a center which includes the pseudoscalars as well as the scalars. If n is odd and then, just as in the complex case, the algebra Clp,q decomposes into a direct sum of isomorphic algebras
each of which is central simple and so isomorphic to matrix algebra over R or H.
If n is odd and then the center of Clp,q is isomorphic to C and can be considered as a complex algebra. As a complex algebra, it is central simple and so isomorphic to a matrix algebra over C.

Classification

All told there are three properties which determine the class of the algebra Clp,q:
Each of these properties depends only on the signature modulo 8. The complete classification table is given below. The size of the matrices is determined by the requirement that Clp,q have dimension 2p+q.
p−q mod 8ω2Clp,q
p−q mod 8ω2Clp,q
0+M1+M⊕M
2−M3−M
4+M5+M⊕M
6−M7−M

It may be seen that of all matrix ring types mentioned, there is only one type shared between both complex and real algebras: the type M. For example, Cl2 and Cl3,0 are both determined to be M2. It is important to note that there is a difference in the classifying isomorphisms used. Since the Cl2 is algebra isomorphic via a C-linear map, and Cl3,0 is algebra isomorphic via an R-linear map, Cl2 and Cl3,0 are R-algebra isomorphic.
A table of this classification for follows. Here runs vertically and runs horizontally.

Symmetries

There is a tangled web of symmetries and relationships in the above table.
Going over 4 spots in any row yields an identical algebra.
From these Bott periodicity follows:
If the signature satisfies then
Thus if the signature satisfies,