Kernels of block Hankel operators and independency of vector-valued functions modulo Nevanlinna class

Abstract

For a matrix-valued function $\Phi\in L^2_{M_{n\times m}}$, it is well-known that the kernel of a block Hankel operator $H_\Phi$ is an invariant subspace for the shift operator. Thus, if the kernel is nontrivial, then $\ker H_\Phi= \Theta H^2_{\mathbb C^r}$ for a natural number $r$ and an $m\times r$ matrix inner function $\Theta$ by Beurling-Lax-Halmos Theorem. It will be shown that the size of the matrix inner function $\Theta$ associated with the kernel of a block Hankel operator $H_\Phi$ is closely related with a certain independency of the columns of $\Phi$, which is defined in this paper. As an important application of this result, the shape of shift invariant, or, backward shift invariant subspaces of $H^2_{\mathbb C^n}$ generated by finite elements will be studied.