Factorizations of PSD Matrix Polynomials and their Smith Normal Forms

Algebra Seminar
Monday, February 20, 2017 - 3:00pm for 1 hour (actually 50 minutes)
Skiles 006
Christoph Hanselka – University of Auckland
Greg Blekherman
It is well-known, that any univariate polynomial matrix A over the complex numbers that takes only positive semidefinite values on the real line, can be factored as A=B^*B for a polynomial square matrix B. For real A, in general, one cannot choose B to be also a real square matrix. However, if A is of size nxn, then a factorization A=B^tB exists, where B is a real rectangular matrix of size (n+1)xn. We will see, how these correspond to the factorizations of the Smith normal form of A, an invariant not usually associated with symmetric matrices in their role as quadratic forms. A consequence is, that the factorizations canusually be easily counted, which in turn has an interesting application to minimal length sums of squares of linear forms on varieties of minimal degree.