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Vestnik Sankt-Peterburgskogo Universiteta. Seriya 10. Prikladnaya Matematika. Informatika. Protsessy Upravleniya, 2011, Issue 4, Pages 63–72
(Mi vspui59)
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Applied mathematics
Schur’s rational approximation of Schur’s functions
V. S. Mikheev St. Petersburg State University, Faculty of Applied Mathematics and Control Processes
Abstract:
The problem of approximating elements from the class $H_2^+$ of the analytic functions in the closed unit disk $U$ assuming only real values on the segment [0,1] is investigated. As approximant class is taken to be ${\mathcal H}_{n}^{+}$ which is the class of irreducible real rational functions with the degrees of a numerator and a denominator not greater $n$. It is proved that if $f\in H_2^+$ and $f\notin {\mathcal H}_{k}^{+}$ where $k<n$ then any local minimizer of nonlinear programme $\displaystyle \|{f-g}\|^2\longrightarrow \min_{g\in {\mathcal H}_{n}^{+}}$ does not belong to ${\mathcal H}_{m}^{+}$, where $m<n$. The result is expanded to the class $S^+$ of Schur's functions selected from $H_2^+$ by the condition $\sup_{z\in U} |f(z)|\leq 1$. If ${\mathcal S}_n^+$ is a Schur's subclass of ${\mathcal H}_{n}^{+}$ then it is proved that, when $f\in S^+$ and $f\notin {\mathcal S}_{k}^{+}$, where $k<n$, any local minimizer of non linear programme $\displaystyle \|{f-g}\|^2\longrightarrow \min_{g\in {\mathcal S}_{n}^{+}}$ does not belong to ${\mathcal S}_{m}^{+}$, where $m<n$.
Keywords:
unit disk, Schur’s function, approximation, rational function, Schur’s algorithm.
Accepted: May 19, 2011
Citation:
V. S. Mikheev, “Schur’s rational approximation of Schur’s functions”, Vestnik S.-Petersburg Univ. Ser. 10. Prikl. Mat. Inform. Prots. Upr., 2011, no. 4, 63–72
Linking options:
https://www.mathnet.ru/eng/vspui59 https://www.mathnet.ru/eng/vspui/y2011/i4/p63
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