Research Article | Open Access

# Higher-Order Hermite-Fejér Interpolation for Stieltjes Polynomials

**Academic Editor:**Qiankun Song

#### Abstract

Let and be the ultraspherical polynomials with respect to . Then, we denote the Stieltjes polynomials with respect to satisfying . In this paper, we consider the higher-order Hermite-Fejér interpolation operator based on the zeros of and the higher order extended Hermite-Fejér interpolation operator based on the zeros of . When is even, we show that Lebesgue constants of these interpolation operators are and , respectively; that is, and . In the case of the Hermite-Fejér interpolation polynomials for , we can prove the weighted uniform convergence. In addition, when is odd, we will show that these interpolations diverge for a certain continuous function on , proving that Lebesgue constants of these interpolation operators are similar or greater than log .

#### 1. Introduction

Let and
For any real-valued function on and an integer , we recall that there exist unique *Hermite and Hermite-Fejér interpolatory polynomials of higher order* denoted by , and of degree , defined as follows:
We note that, by definition, is the Lagrange, is the Hermite-Fejér, and is the Krylov-Stayermann interpolatory polynomial. By (2), we may write
The polynomials
are unique, of degree exactly and satisfy the relations
where for nonnegative integers and
Here, are the well-known fundamental Lagrange polynomials of degree given by
and the coefficients may be obtained from the relations
If , then the Hermite interpolation polynomial of degree with respect to is defined by
We may express as
where for
is the unique polynomial of degree satisfying
Then, we easily see from the relations (5) and (12) that , , and for and (see [1]). Now, we have for any polynomial of degree ,
In what follows, we abbreviate several notations as , , and if there is no confusion. Here, we are interested in Hermite-Fejér and Hermite interpolations with respect to whose elements are the zeros of a sequence of Stieltjes polynomials and the product polynomials of Stieltjes polynomials and the ultraspherical polynomials, respectively. To be precise, we first consider the generalized Stieltjes polynomials defined (up to a multiplicative constant) by
where , , and is the th ultraspherical polynomial for the weight function . In 1935, Szegö [2] showed that the zeros of the generalized Stieltjes polynomials are real and inside and interlace with the zeros of whenever .

For the properties of interpolation operators based at the zeros of and the zeros of , Ehrich and Mastroianni [3, 4] proved that Lagrange interpolation operators based on the zeros of and extended Lagrange interpolation operators based on the zeros of have Lebesgue constants and of optimal order, that is, . For the Hermite-Fejér interpolation operator based on the zeros of and the extended Hermite-Fejér interpolation operator based on the zeros of , it is proved that Lebesgue constants and are of optimal order, that is, , in [5]. In this paper, we consider the higher-order Hermite-Fejér interpolation operator based on the zeros of and the higher-order extended Hermite-Fejér interpolation operator based on the zeros of . When is even, we show that Lebesgue constants of these interpolation operators are and , respectively; that is, and . In the case of the Hermite-Fejér interpolation polynomials for , we can prove the weighted uniform convergence. In addition, when is odd, we will show that these interpolations diverge for a certain continuous function on , proving that Lebesgue constants of these interpolation operators are similar or greater than .

This paper is organized as follows. In Section 2, we will introduce the main results. In Section 3, we will show the auxiliary propositions and estimate the coefficients of Hermite-Fejér interpolation polynomials in order to prove the main results. Finally, we will prove the results in Section 4.

#### 2. Main Results

We first introduce some notations, which we use in the following. For the ultraspherical polynomials , , we use the normalization . We denote the zeros of by , , and the zeros of Stieltjes polynomials by , . We denote the zeros of by , . All nodes are ordered by increasing magnitude. We set , and for any two sequences and of nonzero real numbers (or functions), we write if and . We denote the space of polynomials of degree at most by .

For the Chebyshev polynomial , note that for and In this paper, we let . In these cases, the results are well known or can easily be deduced. Therefore, we will consider the cases for and let in the following.

Let be the Hermite-Fejér interpolation polynomials of with respect to the zeros of . Also let be the Hermite-Fejér interpolation polynomials of with respect to the zeros of . The fundamental Lagrange interpolation polynomials and with respect to and , respectively, are given by

We let and be the Lebesgue constants based on the zeros of and , respectively. That is, the Lebesgue constants and are defined as follows: and for a nonnegative real function , where and are the coefficients of the higher-order Hermite-Fejér interpolation polynomials defined in (4), with respect to and , respectively.

##### 2.1. Uniform Convergence of Hermite-Fejér Interpolation Polynomials of Higher Order

Theorem 1. *Let and . *(a)*
*(b)*Suppose that . Then, for a continuous function on one has uniformly for ,
*

Theorem 2. *Let and .*(a)* Then one has for * *and for *(b)* For a continuous function on , if , then one has uniformly for ,
* *and if , then
*

##### 2.2. Divergence of Hermite-Fejér Interpolation Polynomials of Higher Order

Theorem 3. *Let and . Then,
*

Theorem 4. *Let and . Let . Suppose that
**
If , then
*

Theorem 5. *Let and . Then,
*

Theorem 6. *Let and . Let . Suppose that
**
If , then
*

If the Lebesgue constant is not bounded, then we know from Helley’s theorem that Hermite-Fejér interpolation does not converge for a certain continuous function on .

#### 3. Estimation of the Coefficients of Higher-Order Hermite-Fejér Interpolation Polynomials

Proposition 7. *Let .*(1)*See [4, Theorem *2.1],* for *,
*Furthermore, *.(2)*See [4, Theorem 2.1] and *[6, ], *for *,

Proposition 8 (see [4, Lemma 5.5]). *Let . Then, for ,
**
and for ,
*

Proposition 9 (see [7, Theorem 2.1]). *Let and a positive integer. Then, for all ,
**
Moreover, one has for ,
*

Proposition 10 (see [7, Theorem 2.2]). *Let and a positive integer. Then, for all ,
**
Moreover, one has for ,
*

Proposition 11 (see [7, Theorems 2.3, 2.4]). *Let and an even integer.*(a)*For *,
(b)*For *,

Proposition 12 (see [7, Lemma 4.9]). *For , one has
*

Proposition 13 (see [7, Theorems 2.6, 2.7]). *Let and .*(a)*For ** and a positive integer *, *one has*(b)*For ** and a positive integer *, *one has**where *.

Theorem 14. *Let , and .*(a)* Uniformly for
and if is odd,
*(b)* Uniformly for ,
and if is odd,
*

Theorem 15. *Let , and .*(a)* Uniformly for ,
and if is odd, one has
*(b)* Uniformly for ,
and if is odd,
*

Theorem 16. *Let and . Suppose that . Then, there exists a constant depending only on and such that
*

Theorem 17. *Let and . Suppose that . Then, there exists a constant with depending only on and such that
*

Theorem 18. *Let and . Suppose that . Then, there exists a constant depending only on and such that
*

Theorem 19. *Let and . Suppose that . Then, there exists a constant with depending only on and such that
*

*Proof of Theorem 14. *We prove by induction on . Since
we know that
So, it holds for by (34) and (36). Now, assume that it holds for . Then, using Leibnitz’s rule for differentiation, we obtain
Suppose that is odd. Then,
Since is odd for an even and is even for an odd , we have by the mathematical induction on , (34), (36), (40), and (58),
These complete the proofs of (45) and (46). To prove (47) and (48), we proceed by induction on . Firstly, for , (47) is trivial since . For , we have by [8, ] and [8, ]
so that
Thus, if we assume that (47) holds for , , then by (45), we have
Suppose that is odd. Then,
Since is odd for an even and is even for an odd , we have by the mathematical induction, (45), (46), and (47)
These complete the proofs of (47) and (48).

*Proof of Theorem 15. *Using (35), (38), and (41), this is proved by the same method as the proof of Theorem 14.

For define and for We rewrite the relation (67) in the form for , and for , , Now, for every , we will introduce an auxiliary polynomial determined by as the following lemma.

Lemma 20 (see [9, Lemma 11]). (i)*For , there exists a unique polynomial of degree such that
*(ii)* and , .*

Lemma 21 (see [9, Lemma 13]). *If , then for ,
*

Lemma 22 (see [9, Proof of Lemma 14]). *For positive integers and ,
*

*Proof of Theorem 16. *Similarly to Theorem 14, we use mathematical induction with respect to . From (58), (43), and (34), we know that
Then, from the following relations: