Appendix A A Matrix Representation of a Clifford Algebra
To construct a matrix representation for a Clifford algebra corresponding to a Euclidean or pseudo-Euclidean space, it is not sufficient to construct a set fe1; e2;:::;eng such that
ej ek C ekej D 2njkI, where (A.1)
nkk D hek; eki D 1 for k D 1;2;:::;p,
nkk D hek; eki D 1 for k D p C 1; p C 2;:::;pC q D n,and ˝ ˛ njk D ej ; ek D 0 for j ¤ k.
We must also consider all possible products of the form M1M2:::Mn where n Mk D ek or I. These products form a set of 2 matrices that should be linearly independent. For our purposes, this set should be linearly independent. One’s first thought is that to check that these products are linearly independent would be a formidable task. However due to a theorem proven by Ian Porteus, there is a simple test to check for this required linear independence (Porteus 1981, pp. 243–245).
Theorem 291. Asetfe1; e2;:::;eng consisting of n matrices representing or- thonormal 1-vectors generates a vector space (and therefore an algebra) of n dimension 2 unless the product e1e2 :::en D e12:::n is a scalar multiple of I. Proof. With one exception, a product of one or more distinct orthonormal 1-vectors will anticommute with at least one of the 1-vectors. To see this, we note that a product of an even number of the 1-vectors will anticommute with any 1-vector appearing in the product. We also see that the product of an odd number of Dirac matrices will anticommute with any Dirac matrix that does not appear in the product. The one product that commutes with all the 1-vectors in the set is the product e12:::n where n is odd. The proof now proceeds by self-contradiction. Suppose the products are not linearly independent. In that case, there exists a set of coefficients Aj1j2:::jk (not all zero) such that
J. Snygg, A New Approach to Differential Geometry using Clifford’s Geometric Algebra, 431 DOI 10.1007/978-0-8176-8283-5, © Springer Science+Business Media, LLC 2012 432 A A Matrix Representation of a Clifford Algebra
Xn X j1j2:::jk A ej1j2:::jk D 0.(A.2) kD0 j1