Piecewise Certificates of Positivity for matrix polynomials

We show that any symmetric positive definite homogeneous matrix polynomial M ∈ R[x1, . . . , xn] admits a piecewise semi-certificate, i.e. a collection of identites M(x) = P j fi,j(x)Ui,j(x) T Ui,j(x) where Ui,j(x) is a matrix polynomial and fi,j(x) is a non negative polynomial on a semialgebraic subset Si, where R = ∪ri=1Si. This result generalizes to the setting of biforms. Some examples of certificates are given and among others, we study a variation around the Choi counterexample of a positive semi-definite biquadratic form which is not a sum of squares. As a byproduct we give a representation of the famous non negative sum of squares polynomial xz + zy + yx − 3 xyz as the determinant of a positive semi-definite quadratic matrix polynomial.