Orthogonal Rational Functions by Adhemar Bultheel, Pablo Gonzalez-Vera, Erik Hendriksen, Olav

By Adhemar Bultheel, Pablo Gonzalez-Vera, Erik Hendriksen, Olav Njastad

This quantity generalizes the classical conception of orthogonal polynomials at the complicated unit circle or at the actual line to orthogonal rational features whose poles are between a prescribed set of advanced numbers. the 1st half treats the case the place those poles are all outdoor the unit disk or within the decrease part airplane. Classical subject matters corresponding to recurrence family, numerical quadrature, interpolation homes, Favard theorems, convergence, asymptotics, and second difficulties are generalized and taken care of intimately. an identical subject matters are mentioned for a unique state of affairs the place the poles can be found at the unit circle or at the prolonged genuine line. within the final bankruptcy, numerous functions are pointed out together with linear prediction, Pisarenko modeling, lossless inverse scattering, and community synthesis. This thought has many purposes either in theoretical actual and intricate research, approximation concept, numerical research, method thought, and electric engineering.

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4. The 2 x 2 J-unitary and J-contractive matrix functions with entries in the Nevanlinna class will be important when we develop the recurrence relations for the kernels and the orthogonal rational functions. 5. 1. Hardy classes We shall be concerned with complex function theory on the unit circle and the upper half plane. The complex number field is denoted by C We use the following notation for the unit circle, the open unit disk, and the complement 15 16 L Preliminaries of the closed unit disk: T = {z:\z\ = 1}, B = {z:\z\< 1}, E = {z : \z\ > 1}.

In the more general case where some of the ak do coincide, the Gram matrix looks more complicated and involves derivatives of Q^ evaluated at the at. To see this, we give a technical lemma first. 2. Consider the Riesz-Herglotz kernel D(z, w) = (z+w)(z — w)~l for the disk. Then z, w) = 2(k\)z(z - w)-(M\ k > 1, where 3^ denotes the £th derivative with respect to w. We also have [dkwD(z, w)]m = 2(k\)zk(l-wzy(k+1\ k > 1, where the substar transform is with respect to z. Furthermore, if Q^(w) = i I m ^ ( 0 ) + D(t,w)d/jL(t), then for k > 1 QS£\w) = dkw^(w) = JdkwD(t, w)dn{t) = and (substar with respect to t).

Working this out gives 1 - da > 0, where a — (0\u + 012*)"1 (#21* + 022*) and with equality on 9©. This identifies a as an inner function. 5. J-unitary and J-contractive matrices 41 The following theorem describes a simple matrix from the class By. 4. 50). Proof. This is a classical result that can be found, for example, in Potapov [180, pp. 187-188]. • A matrix in N2x2 that is J-contractive in O and J-unitary will be called J-inner, a terminology used by Dym [77]. The set of J-inner matrix functions is denoted as Bj = By n Ty while we set Bj = By HTy.

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