Perturbation of orthogonal polynomials on an arc of the unit circle
Journal of Approximation Theory
Journal of Approximation Theory
On the domain of convergence and poles of complex J - fractions
Journal of Approximation Theory
Singular measures on the unit circle and their reflection coefficients
Journal of Approximation Theory
Limit points of eigenvalues of truncated tridiagonal operators
Journal of Computational and Applied Mathematics - Special issue on orthogonal polynomials, special functions and their applications
Zeros of orthogonal polynomials on the real line
Journal of Approximation Theory
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In this paper, we obtain new results about the orthogonality measure of orthogonal polynomials on the unit circle, through the study of unitary truncations of the corresponding unitary multiplication operator, and the use of the five-diagonal representation of this operator. Unitary truncations on subspaces with finite co-dimension give information about the derived set of the support of the measure under very general assumptions for the related Schur parameters (a"n). Among other cases, we study the derived set of the support of the measure when lim"n|a"n"+"1/a"n|=1, obtaining a natural generalization of the known result for the Lopez class lim"na"n"+"1/a"n@?T, lim"n|a"n|@?(0,1). On the other hand, unitary truncations on subspaces with finite dimension provide sequences of unitary five-diagonal matrices whose spectra asymptotically approach the support of the measure. This answers a conjecture of L. Golinskii concerning the relation between the support of the measure and the strong limit points of the zeros of the para-orthogonal polynomials. Finally, we use the previous results to discuss the domain of convergence of rational approximants of Caratheodory functions, including the convergence on the unit circle.