Point evaluation and Hardy space on a homogeneous tree

We consider transfer functions of time--invariant systems as defined by Basseville, Benveniste, Nikoukhah and Willsky when the discrete time is replaced by the nodes of an homogeneous tree. The complex numbers are now replaced by a C*-algebra built from the structure of the tree. We define a point evaluation with values in this C*-algebra and a corresponding ``Hardy space'' in which a Cauchy's formula holds. This point evaluation is used to define in this context the counterpart of classical notions such as Blaschke factors. There are deep analogies with the non stationary setting as developed by the first author, Dewilde and Dym.Comment: Added references, changed notation

Discrete Analytic Schur Functions

We introduce the Schur class of functions, discrete analytic on the integer lattice in the complex plane. As a special case, we derive the explicit form of discrete analytic Blaschke factors and solve the related basic interpolation problem

On discrete analytic functions: Products, Rational Functions, and some Associated Reproducing Kernel Hilbert Spaces

We introduce a family of discrete analytic functions, called expandable discrete analytic functions, which includes discrete analytic polynomials, and define two products in this family. The first one is defined in a way similar to the Cauchy-Kovalevskaya product of hyperholomorphic functions, and allows us to define rational discrete analytic functions. To define the second product we need a new space of entire functions which is contractively included in the Fock space. We study in this space some counterparts of Schur analysis

A new realization of rational functions, with applications to linear combination interpolation

We introduce the following linear combination interpolation problem (LCI): Given $N$ distinct numbers $w_1,\ldots w_N$ and $N+1$ complex numbers $a_1,\ldots, a_N$ and $c$, find all functions $f(z)$ analytic in a simply connected set (depending on $f$) containing the points $w_1,\ldots,w_N$ such that $$\sum_{u=1}^Na_uf(w_u)=c.$$ To this end we prove a representation theorem for such functions $f$ in terms of an associated polynomial $p(z)$. We first introduce the following two operations, $(i)$ substitution of $p$, and $(ii)$ multiplication by monomials $z^j, 0\le j < N$. Then let $M$ be the module generated by these two operations, acting on functions analytic near $0$. We prove that every function $f$, analytic in a neighborhood of the roots of $p$, is in $M$. In fact, this representation of $f$ is unique. To solve the above interpolation problem, we employ an adapted systems theoretic realization, as well as an associated representation of the Cuntz relations (from multi-variable operator theory.) We study these operations in reproducing kernel Hilbert space): We give necessary and sufficient condition for existence of realizations of these representation of the Cuntz relations by operators in certain reproducing kernel Hilbert spaces, and offer infinite product factorizations of the corresponding kernels

Schur Analysis and Discrete Analytic Functions: Rational Functions and Co-isometric Realizations

We define and study rational discrete analytic functions and prove the existence of a coisometric realization for discrete analytic Schur multipliers

Discrete analytic Schur functions

We introduce the Schur class of functions, discrete analytic on the integer lattice in the complex plane. As a special case, we derive the explicit form of discrete analytic Blaschke factors and solve the related basic interpolation problem

Relative reproducing kernel Hilbert spaces

We introduce a reproducing kernel structure for Hilbert spaces of functions where differences of point evaluations are bounded. The associated reproducing kernels are characterized in terms of conditionally negative functions