Simple theorem on Hermitian matrices and an application to the polarization of vector particles

It is proven that a necessary and sufficient condition for an n-dimensional Hermitian matrix to be positive definite is that it be expressible in the form =OEO, where O is a complex orthogonal matrix and E is a diagonal matrix with positive elements. This accomplishes a parametrization since O has n2-n real parameters and E has n of them. The proof is constructive, giving O and E. It is further shown that the limit forms of this expression yield all the non-negative definite matrices. The parametrization for the polarization matrix of a spin-one particle is given explicitly.