Asymptotics of matrix orthogonal polynomials with non-scalar varying weights

Abstract

We study the $2\times 2$ matrix orthogonal polynomials ${\mathbf{P}}_N$ on the real line with respect to the varying weights ${\mathrm{e}}^{-N\mathbf{Q}(z)}\mathbf{B}(z)$, where $\mathbf{Q}$ and $\mathbf{B}$ are real symmetric matrix polynomials that commute with a Hermite-type matrix weight, $\mathbf{Q}$ is not scalar and $\mathbf{B}$ is positive definite. We are motivated by the non-Abelian Toda deformation of the Hermite-type weight, in the sense of Deaño, Morey and Román. The weight depends on $N$ through a matrix exponential that is not scalar, so that its two eigenvalues have different exponential rates. The matrix orthogonality reduces to two scalar orthogonality problems on the real line, with a varying weight that has an essential singularity at the origin. Assuming that the associated equilibrium measure is one-cut regular, we obtain the strong asymptotics of ${\mathbf{P}}_N$ and of $\det {\mathbf{P}}_N$ by the Deift-Zhou steepest descent method. Our main result is the limiting zero distribution of the entries of ${\mathbf{P}}_N$. These zeros are complex in general, and they accumulate on a curve where the contributions of the two eigenvalues of the weight have the same size. When this curve crosses the real segment that attracts the zeros of $\det {\mathbf{P}}_N$, the zeros of the entries display a critical behavior, and a positive fraction of them accumulates on this segment.

arXiv comment47 pages, 1 figure