121 research outputs found
Polynomial interpolation over quaternions
Interpolation theory for complex polynomials is well understood. In the
non-commutative quaternionic setting, the polynomials can be evaluated "on the
left" and "on the right". If the interpolation problem involves interpolation
conditions of the same (left or right) type, the results are very much similar
to the complex case: a consistent problem has a unique solution of a low degree
(less than the number of interpolation conditions imposed), and the solution
set of the homogeneous problem is an ideal in the ring . The
problem containing both "left" and "right" interpolation conditions is quite
different: there may exist infinitely many low-degree solutions and the
solution set of the homogeneous problem is a quasi-ideal in
An algorithm for finding low degree rational solutions to the Schur coefficient problem
We present an algorithm producing all rational functions with prescribed
Taylor coefficients at the origin and such that and
for every fixed . The case where is also discussed
Confluent Vandermonde matrices, divided differences, and Lagrange-Hermite interpolation over quaternions
We introduce the notion of a confluent Vandermonde matrix with quaternion
entries and discuss its connection with Lagrange-Hermite interpolation over
quaternions. Further results include the formula for the rank of a confluent
Vandermonde matrix, the representation formula for divided differences of
quaternion polynomials and their extensions to the formal power series setting
Pick matricies and quaternionic power series
It is well known that a non-constant complex-valued function defined on
the open unit disk is an analytic self-mapping of \D if and only
if Pick matrices are
positive semidefinite for all choices of finitely many points z_i\in\D. A
stronger version of the "if" part was established by Alan Hindmarsh: if all
Pick matrices are positive semidefinite, then is an analytic
self-mapping of . In this paper, we extend this result to the
non-commutative setting of power series over quaternions
Zeros and factorizations of quaternion polynomials: the algorithmic approach
It is known that polynomials over quaternions may have spherical zeros and
isolated left and right zeros. These zeros along with appropriately defined
multiplicities form the zero structure of a polynomial. In this paper, we
equivalently describe the zero structure of a polynomial in terms of its left
and right spherical divisors as well as in terms of left and right
indecomposable divisors. Several algorithms are proposed to find left/right
zeros and left/right spherical divisors of a quaternion polynomial, to
construct a polynomial with prescribed zero structure and more generally, to
construct the least left/right common multiple of given polynomials. Similar
questions are briefly discussed in the setting of quaternion formal power
series
A uniqueness result on boundary interpolation
Let be an analytic function mapping the unit disk \D to itself. We give
necessary and sufficient conditions on the local behavior of near a finite
set of boundary points that requires to be a finite Blaschke product
On the Sylvester matrix equation over quaternions
The Sylvester equation is considered in the setting of quaternion
matrices. Conditions that are necessary and sufficient for the existence of a
unique solution are well-known. We study the complementary case where the
equation either has infinitely many solutions or does not have solutions at
all. Special attention is given to the case where and are respectively,
lower and upper triangular two-diagonal matrices (in particular, if and
are Jordan blocks
Boundary Nevanlinna--Pick interpolation problems for generalized Schur functions
Three boundary Nevanlinna-Pick interpolation problems at finitely many points
are formulated for generalized Schur functions. For each problem, the set of
all solutions is parametrized in terms of a linear fractional transformation
with a Schur class parameter
The higher order Carath\'eodory--Julia theorem and related boundary interpolation problems
The higher order analogue of the classical Carath\'eodory-Julia theorem on
boundary angular derivatives has been obtained in \cite{bk3}. Here we study
boundary interpolation problems for Schur class functions (analytic and bounded
by one in the open unit disk) motivated by that result
de Branges-Rovnyak spaces and norm-constraint interpolation
For a contractive analytic operator-valued function on the unit disk
, de Branges and Rovnyak associate a Hilbert space of analytic
functions . A companion survey provides equivalent definitions
and basic properties of these spaces as well as applications to function theory
and operator theory. The present survey brings to the fore more recent
applications to a variety of more elaborate function theory problems, including
-norm constrained interpolation, connections with the Potapov method
of Fundamental Matrix Inequalities, parametrization for the set of all
solutions of an interpolation problem, variants of the Abstract Interpolation
Problem of Katsnelson, Kheifets, and Yuditskii, boundary behavior and boundary
interpolation in de Branges-Rovnyak spaces themselves, and extensions to
multivariable and Kre\u{\i}n-space settings
- β¦