Analytic approximation of rational matrix functions

Abstract

For a rational matrix function $\Phi$ with poles outside the unit circle, we estimate the degree of the unique superoptimal approximation $\A\Phi$ by matrix functions analytic in the unit disk. We obtain sharp estimates in the case of $2\times2$ matrix functions. It turns out that ``generically'' $\deg\A\Phi\le\deg\Phi-2$. We prove that for an arbitrary $2\times2$ rational function $\Phi$, $\deg\A\Phi\le2\deg\Phi-3$ whenever $\deg\Phi\ge2$. On the other hand, for $k\ge2$, we construct a $2\times2$ matrix function $\Phi$, for which $\deg\Phi=k$, while $\deg\A\Phi=2k-3$. Moreover, we conduct a detailed analysis of the situation when the inequality $\deg\A\Phi\le\deg\Phi-2$ can violate and obtain best possible results.