Non-Commutative Partial Matrix Convexity

Abstract

Let $p$ be a polynomial in the non-commuting variables $(a,x)=(a_1,...,a_{g_a},x_1,...,x_{g_x})$. If $p$ is convex in the variables $x$, then $p$ has degree two in $x$ and moreover, $p$ has the form $p=L+{\Lambda }^T\Lambda ,$ where $L$ has degree at most one in $x$ and $\Lambda$ is a (column) vector which is linear in $x,$ so that ${\Lambda }^T\Lambda$ is a both sum of squares and homogeneous of degree two. Of course the converse is true also. Further results involving various convexity hypotheses on the $x$ and $a$ variables separately are presented.

24 pages