The Clifford algebra is the algebra over with generators and relations
The first few Clifford algebras are easy to describe
If is a field, denote by the algebra of all matrices with entries in . We call a full matrix algebra. It turns out that the Clifford algebras are all full matrix algebras or the direct sums of two full matrix algebras:
This table exhibits clearly a periodic pattern of period , except for the dimension increase after each period. The -fold periodicity of the Clifford algebras, long known to algebraists, is reminiscent of the -fold periodicity of the stable homotopy groups of the orthogonal group.
In the early Sixties Michael Atiyah, Raoul Bott, and Arnold Shapiro found an explanation for this tantalizing connection. The link is provided by a class of linear differential operators called the Dirac operators. The link between differential equations and homotopy groups first came about as a result of the realization that ellipticity of a differential operator can be defined in terms of the symbol of the differential operator.
Suppose we can find real matrices of size satisfying
This corresponds to a real representation of the Clifford algebra . The associated Dirac operator is the linear first-order differential operator
where is the identity matrix. Such a differential operator on has a symbol obtained by replacing by a variable :
The Dirac operator is readily shown to be elliptic; this means its symbol is nonsingular for all in . Therefore, when restricted to the unit sphere in , the symbol of the Dirac operator gives a map
Since has the homotopy type of , this map given by the symbol of the Dirac operator defines an element of the homotopy group .
The paper  shows that the minimal-dimensional representations of the Clifford algebras give rise to Dirac operators whose symbols generate the stable homotopy groups of the orthogonal group. In this way, the -fold periodicity of the Clifford algebras reappears as the -fold periodicity of the stable homotopy groups of the orthogonal group.