Indefinite determinantal representations versus nonsingularities on the noncommutative d-torus

Abstract

We show that for a multivariable polynomial $p(z)=p(z_1,\dots ,z_d)$ with a determinantal representation $$p(z)=p(0)\det (I_n-K({\oplus }_{j=1}^d{z}_j{I}_{n_j}))$$ the matrix $K$ is structurally similar to a strictly $J$-contractive matrix for some diagonal signature matrix $J$ if and only if the extension of $p(z)$ to a polynomial in $d$-tuples of matrices of arbitrary size given by \[ p(U_1, \ldots , U_d) = p(0,\ldots,0) \det (I_n\otimes I_m- (K\otimes I_m) (\oplus_{j=1}^d I_{n_j}\otimes U_j)), \] where $U_1,\dots ,U_d\in {\mathbb{C}}^{m\times m}$, $m\in \mathbb{N}$, does not have roots on the noncommutative $d$-torus consisting of $d$-tuples $(U_1,\dots ,U_d)$ of unitary matrices of arbitrary size.

9 pages