Hermite-Fejér type interpolation of higher order
Let \(X:= \{x_{nk}, y_{n\mu}\}\) be a matrix of nodes satisfying \[ -1\leq x_{n1}<\cdots< x_{nn}\leq 1,\quad -1\leq y_{n1}<\cdots< y_{n\ell}\leq 1, \] where \(x_{nk}\neq y_{n\mu}\), \(k= 1,\dots, n\), \(\mu= 1,\dots,\ell\), \(n\geq 1\), \(\ell= \ell(n)\geq 0\). Let \(\pi_n\) denote the set of polynomials of degree \(\leq n\). If \(f\in C^{n-1}[-1, 1]\), let \(S^*_{mn}(f)\) denote Hermite type polynomials of higher order satisfying \[ \begin{aligned} S^*_{mn}(f)\in \pi_{mn+ \ell-1},\;S^{*(\nu)}_{m,n}(f, x_k) & = f^{(\nu)}(x_k),\;k=1,\dots, n;\;\nu= 0,1,\dots,m- 1,\\ S^*_{mn}(f, y_\mu) & = f(y_\mu),\;\mu= 1,\dots,\ell.\end{aligned} \] Similarly let \(S_{mn}(f)\) denote Hermite-Fejér type interpolation polynomials satisfying \[ \begin{aligned} S_{mn}(f) & \in \pi_{mn+ \ell-1},\;S_{mn}(f, x_k)= f(x_k),\quad k= 1,\dots,n,\\ S^{(\nu)}_{mn}(f, x_k) & = 0,\quad k= 1,\dots, n,\quad \nu= 1,\dots, m-1,\\ S_{mn}(f, y_\mu) & = f(y_\mu),\quad \mu= 1,\dots,\ell.\end{aligned} \] For \(\ell= 0\), \(S^*_{mn}(f, x)\) is the usual Hermite interpolation polynomial and \(S_{mn}(f, x)\) is the usual Hermite-Fejér interpolant. In case \(\ell= 2\), \(y_{n1}= y_{n2}= 1\), \(S_{mn}(f, x)\) is the quasi-Hermite interpolant. Then one can write \[ \begin{aligned} S^*_{mn}(f, x) & = \sum^n_{i=1} \sum^{m-1}_{j= 0} f^{(j)}(x_i)B_{ij}(x)+ \sum^\ell_{\mu= 1} f(y_\mu)C_\mu(x),\\ S_{mn}(f, x) & = \sum^n_{i= 1}f(x_i) B_{i0}(x)+ \sum^\ell_{\mu= 1} f(y_\mu) C_\mu(x),\end{aligned} \] where \(\{B_{ij}(x)\}^{n,m-1}_{i=1,j= 0}\) and \(\{C_\mu(x)\}\) are the fundamental polynomials of this interpolation problem. Using suitable variations on the method of \textit{J. Szabados} [Acta Math. Hung. 61, 357-368 (1993; Zbl 0801.41001)], the authors find in Section 2, the explicit forms for the fundamental polynomials \(\{B_{ij}(x)\}\) and \(\{C_\mu(x)\}^\ell_{\mu= 1}\) and obtains some estimates for them. The main results are given in Section 5. Let \(R_{mn}(f)= f(x)- S_{mn}(f)\) be the error of approximation of \(f\) by \(S_{mn}(f)\). Then the authors show Proposition 8. For \(p\in \pi_{mn+\ell- 1}\), \[ \|R^{(j- 1)}_{mn}(p)\|\geq (mn+ \ell-1)^{-1} \sup_i (1- x^2_{ni})^{1/2}|p^{(j)}(x_{ni})|,\quad j= 1,\dots, n-1. \] From this the authors prove the following saturation theorem for Hermite-Fejér interpolation: Proposition 9. If \(X\) has infinitely many limit points and a polynomial \(p(x)\) satisfies for a fixed \(j\) \((1\leq j\leq m-1)\), \[ \|R^{(j)}_{mn}(p)\|= o((mn+ \ell-1)^{-1}) \] then \(p(x)\) must be a polynomial of degree \(j-1\). Proposition 10. Suppose \(\|S_{mn}\|= o(1)\) and \(\overline X:=\text{closure of }X= [-1,1]\). If a function \(f(x)\) is such that \(f^{(j)}\in C\), \(1\leq j\leq m-1\), satisfies \[ \|R_{mn}(f^{(j-1)})\|= o((mn+ \ell-1)^{-1}), \] then \(f(x)\) is a polynomial of degree \(j-1\).
scientific article; zbMATH DE number 1367958