Relative Asymptotic of Multiple Orthogonal Polynomials for Nikishin Systems

We prove relative asymptotic for the ratio of two sequences of multiple orthogonal polynomials with respect to Nikishin system of measures. The first Nikishin system ${\mathcal{N}}(\sigma_1,...,\sigma_m)$ is such that for each $k$, $\sigma_k$ has constant sign on its compact support \supp {\sigma_k} \subset \mathbb{R} consisting of an interval $\widetilde{\Delta}_k$, on which $|\sigma_k^{\prime}| > 0$ almost everywhere, and a discrete set without accumulation points in $\mathbb{R} \setminus \widetilde{\Delta}_k$. If {Co}(\supp {\sigma_k}) = \Delta_k denotes the smallest interval containing \supp {\sigma_k}, we assume that $\Delta_k \cap \Delta_{k+1} = \emptyset$, $k=1,...,m-1$. The second Nikishin system ${\mathcal{N}}(r_1\sigma_1,...,r_m\sigma_m)$ is a perturbation of the first by means of rational functions $r_k$, $k=1,...,m,$ whose zeros and poles lie in $\mathbb{C} \setminus \cup_{k=1}^m \Delta_k$.Comment: 30 page

Determining radii of meromorphy via orthogonal polynomials on the unit circle

19 pages, no figures.-- MSC2000 codes: 30E10, 42C05, 41A20, 30D30.MR#: MR2016676 (2004k:30087)Zbl#: Zbl 1051.30033Using a convergence theorem for Fourier–Padé approximants constructed from orthogonal polynomials on the unit circle, we prove an analogue of Hadamard's theorem for determining the radius of m-meromorphy of a function analytic on the unit disk and apply this to the location of poles of the reciprocal of Szegö functions.The research of D.B.R. and G.L.L. was supported, in part, by Dirección General de Investigación, Ministerio de Ciencia y Tecnología, under grant BFM 2000-0206-C04-01 and the research of G.L.L. was also supported by Ministerio da Ciencia e do Ensino Superior, under Grant PRAXIS XXI BCC-22201/99, and by INTAS under Grant 2000-272. The research of E.B.S. was supported, in part, by V.S. National Science Foundation Grant DMS-0296026.Publicad

Multipoint rational approximants with preassigned poles

20 pages, no figures.-- MSC1991 codes: 41A21, 42C05, 30E10.MR#: MR1820073 (2002i:41021)Zbl#: Zbl 1160.41305Let $\mu$ be a finite positive Borel measure whose support $S(\mu)$ is a compact regular set contained in $\Bbb R$. For a function of Markov type $\hat\mu(z)=\int_{S(\mu)}d\mu(x)/(z-x)$, z\in\Bbb C\sbs S(\mu), we consider multipoint Padé-type approximants (MPTAs), where some poles are preassigned and interpolation is carried out along a table of points contained in \overline{\Bbb C}\sbs {\rm Co}(S(\mu)) which is symmetrical with respect to the real line. The main purpose of this paper is the study of the `exact rate of convergence' of the MPTAs to the function $\hat\mu$.Research by first author (F.C.) partially carried out at the Mathematics Department of Umeå University under Guest Scholarship from the Swedish Institute. Research by second author (G.L.L.) partially supported by Dirección General de Enseñanza Superior under grant PB 96-0120-CO3-01 and by INTAS under grant 93-0219 EXT.Publicad

Direct and Inverse Results for Multipoint Hermite-Pade Approximants

Given a system of functions f = (f1, . . . , fd) analytic on a neighborhood of some compact subset E of the complex plane, we give necessary and sufficient conditions for the convergence with geometric rate of the common denominators of multipoint Hermite-Pade approximants. The exact rate of convergence of these denominators and of the approximants themselves is given in terms of the analytic properties of the system of functions. These results allow to detect the location of the poles of the system of functions which are in some sense closest to E.Comment: 18 pages. arXiv admin note: text overlap with arXiv:1606.07920, arXiv:1801.03004, arXiv:1203.494