Research Article  Open  Published:
JacobiSobolev Orthogonal Polynomials: Asymptotics for NCoherence of Measures
Journal of Inequalities and Applicationsvolume 2011, Article number: 294134 (2011)
Abstract
Let us introduce the Sobolevtype inner product , where and , , with and for all A MehlerHeinetype formula and the inner strong asymptotics on as well as some estimates for the polynomials orthogonal with respect to the above Sobolev inner product are obtained. Necessary conditions for the norm convergence of Fourier expansions in terms of such Sobolev orthogonal polynomials are given.
1. Introduction
For a nontrivial probability measure , supported on , we define the linear space of all measurable functions on such that , where
Let us now introduce the Sobolevtype spaces (see, e.g., [1, Chapter 3] in a more general framework)
where and , with , , , and , for all . We denote by the vector of dimension with components .
Let and in . We can introduce the Sobolevtype inner product
where and
where , , , , and , for all . In the sequel, we will assume that , and, therefore, for all , and .
Using the standard GramSchmidt method for the canonical basis in the linear space of polynomials, we obtain a unique sequence (up to a constant factor) of polynomials orthogonal with respect to the above inner product. In the sequel, they will called JacobiSobolev orthogonal polynomials.
For and , the pair of measures is a 0coherent pair, studied in [2–4] (see also [5] in a more general framework). In [6], the authors established the distribution of the zeros of the polynomials orthogonal with respect to the above Sobolev inner product (1.3) when and . Some results concerning interlacing and separation properties of their zeros with respect to the zeros of Jacobi polynomials are also obtained assuming we are working in a coherent case. More recently, for a noncoherent pair of measures, when , , , and , the distribution of zeros of the corresponding Sobolev orthogonal polynomials as well as some asymptotic results (more precisely, inner strong asympttics, outer relative asymptotics, and MehlerHeine formulas) for these sequences of polynomials are deduced in [7–9]. In the Jacobi case, some analog problems have been considered in [10, 11].
The aim of this contribution is to study necessary conditions for norm convergence of the Fourier expansion in terms of JacobiSobolev orthogonal polynomials. In order to prove it, we need some estimates and strong asymptotics for the polynomials as well as for their derivatives . A MehlerHeinetype formula, inner strong asymptotics, upper bounds in , and norms of JacobiSobolev orthonormal polynomials are obtained. Thus, we extend the results of [10] for generalized coherent pairs of measures.
The structure of the manuscript is as follows. In Section 2, we give some basic properties of Jacobi polynomials that we will use in the sequel. In Section 3, an algebraic relation between the sequences of polynomials and Jacobi orthonormal polynomials is stated. It involves (where ) consecutive terms of such sequences in such a way that we obtain a generalization of the relations satisfied in the coherent case. Upper bounds for the polynomials and their derivatives in are deduced. The inner strong asymptotics as well as a MehlerHeinetype formula are obtained. Finally, the asymptotic behavior of these polynomials with respect to the norm is studied. In Section 4, necessary conditions for the convergence of the Fourier expansions in terms of the sequence of JacobiSobolev orthogonal polynomials are presented.
Throughout this paper, positive constants are denoted by and they may vary at every occurrence. The notation means that the sequence converges to 1 and notation means for sufficiently large .
2. Preliminaries
For , , we denote by the sequence of Jacobi polynomials which are orthonormal on with respect to the inner product
We will denote by the leading coefficient of any polynomial , and . Now, we list some properties of the Jacobi orthonormal polynomials which we will use in the sequel.
Proposition 2.1.

(a)
The leading coefficient of is (see [12, formulas (4.3.4) and (4.21.6)])
(b)The derivatives of Jacobi polynomials satisfy (see [12, formula (4.21.7)])
(c)For , and
where if and if (see [12, Theorem 7.32.1]).
(d)For the polynomials , we get the following estimate (see [12, formula (7.32.6)], [13, Theorem 1]):
where and .
(e)MehlerHeine formula (see [12, Theorem 8.1.1])
where are real numbers and is the Bessel function of the first kind. This formula holds locally uniformly, that is, on every compact subset of the complex plane.
(f)Inner strong asymptotics. For , when and , we get (see [12, Theorem 8.21.8])
where , , , and .
(g)For ,, , and (see [12, p.391. Exercise 91], [14, (2.2)], [15, Theorem 2]),
Let be the sequence of orthonormal polynomials with respect to the inner product (1.5), and let
be the th polynomial orthonormal with respect to , where and , , are the Tchebychev polynomials of the first kind.
Proposition 2.2 ([16, Lemma 2.1]).
For , there exist constants such that
and , where
Next, we will consider the polynomials
where , . Notice that
Taking into account that the zeros of the polynomial orthogonal with respect to on the interval are real, simple, and located in , we have . Therefore, for large enough.
On the other hand, using (b) in Proposition 2.1, we have
From Proposition 2.1 and (2.12), we get the following.
Proposition 2.3.

(a)
For ,, and ,
(215)
where if and if .

(b)
When and , , we get the following estimate for the polynomials :
(216)

(c)
MehlerHeine type formula. We get
(217)
where are real numbers, and is the Bessel function of the first kind. This formula holds locally uniformly, that is, on every compact subset of the complex plane.

(d)
Inner strong asymptotics. When and , we get
(218)
where , , , and .

(e)
For ,, , and ,
(219)
3. Asymptotics of JacobiSobolev Orthogonal Polynomials
Let denote the sequence of polynomials orthogonal with respect to (1.3) normalized by the condition that they have the same leading coefficient as , that is, .
The following relation between and holds.
Proposition 3.1.
For ,
where, for ,
Moreover, and for .
Proof.
Expanding with respect to the basis of the linear space of polynomials with degree at most , we get
where, for ,
For ,
Therefore,
As a conclusion,
Using the extremal property for monic orthogonal polynomials with respect to the corresponding norm (see [12, Theorem 3.1.2]),
we get
Thus,
Finally, from (3.8), we find that
and from Schwarz inequality,
Thus,
Using (3.1) in a recursive way, we get the representation of the polynomial in terms of the elements of the sequence . More precisely we get the following.
Proposition 3.2.
For , , it holds that
where , , and , , . Moreover, for , and for , .
Proof.
Let denote by , , and , , . First, we prove that
where , and, by convention, , .
We will prove (3.16) by induction. When , it is a trivial result. On the other hand, applying (3.1) in a recursive way, we get
Taking into account (3.7), we have . Thus, (3.16) follows for . Now, we assume (3.16) holds for . Again, from (3.1),
Now, we prove that for . For , this follows from (3.7). Since and , for the statement follows by induction. Thus, (3.16) holds for . Now taking in (3.16), we get (3.15).
Finally, we prove that for , and for , . First, the following inequality holds:
. Indeed, for , (3.19) follows from Proposition 3.1 and (3.7). Now, we assume that the relation (3.19) holds for . Thus, for ,
for
for
and for
Therefore, from
the relation (3.19) holds for . As consequence, for and .
Now, we will prove by induction that for and .
The case follows from (3.19). We assume that for and . For ,
and for
Therefore, from
the statement holds for .
Next, we will give some properties of the JacobiSobolev orthogonal polynomials.
Proposition 3.3.

(a)
For the polynomials , we get
(328)
where , and .

(b)
For the polynomials , we get
(329)
where , and , .
Proof.

(a)
Using Proposition 3.2, we have
(330)
Therefore, from Proposition 2.3, the statement follows immediately.
On the other hand, taking into account Proposition 2.1, Proposition 2.2, (2.14), and (3.15), the proof of can be done in a similar way.
Now, we show that, like for the classical Jacobi polynomials, the polynomial attains its maximum in at the endpoints. More precisely,
Proposition 3.4.

(a)
For , , and
(331)
where if and if .

(b)
For , and
(332)
where if and if .
Proof.
Here, we will prove only the case when . The case when can be done in a similar way.

(a)
From Proposition 2.3,
(333)
for and . Therefore, according to (3.30),
for and . From Proposition 3.1, we get
Finally, from Proposition 2.3(a), the statement follows.

(b)
Taking into account Proposition 2.1, Proposition 2.2, (2.14), (3.1), and (3.15), we can conclude the proof in the same way as we did in (a).
Corollary 3.5.
For , ,
and for , ,
where
Proof.
The inequality
holds for , as well as
for . Therefore, from Propositions 3.3 and 3.4, the statement follows immediately.
Next, we deduce a MehlerHeinetype formula for and (see Theorem 4.1 in [10]).
Proposition 3.6.
Uniformly on compact subsets of ,

(a)
(341)

(b)
(342)
where are real numbers, and is the Bessel function of the first kind.
Proof.
To prove the proposition, we use the same technique as in [17].

(a)
Multiplying in (3.1) by , we obtain
(343)
where , and , . Moreover, and for .
Using the above relation in a recursive way as well as the same argument of Proposition 3.2, we have
where , for , and for ,. Thus,
On the other hand, from Proposition 2.3(c), is uniformly bounded on compact subsets of . Thus, for a fixed compact set , there exists a constant , depending only on , such that when ,
Thus, the sequence is uniformly bounded on . As a conclusion,
and from Proposition 2.3(c), we obtain the result.

(b)
Since we have uniform convergence in (3.41), taking derivatives and using a well known property of Bessel functions of the first kind (see [12, formula 1.71.5]), we obtain (3.42).
Now, we give the inner strong asymptotics of on .
Proposition 3.7.
For and ,
where , , , , and .
Proof.
From Proposition 3.3(a), the sequence is uniformly bounded on compact subsets of ; thus, from Proposition 3.1,
Now, using Proposition 2.3(d), the relation (3.48) follows.
Concerning (3.49), it can be obtained in a similar way by using Propositions 2.1(f) and 2.2, (2.14), Propositions 3.1 and 3.3(b).
Now, we can give the sharp estimate for the Sobolev norms of the JacobiSobolev polynomials.
Proposition 3.8.
For and ,
Proof.
Clearly, if , then we get Proposition 3.4(b). Thus, in the proof, we will assume . Since by Proposition 3.2 and(2.14)
where , , and are bounded because of the orthonormality condition, we obtain
where , and .
On the other hand, using (3.30), Minkowski's inequality, and Proposition 2.3(e), we deduce
In the same way as above, we get
Thus, from (3.53), (3.54), and (3.55), we have
Notice that the upper estimate in (3.54) and (3.55) can also be proved using the bounds for JacobiSobolev polynomials given in Corollary 3.5.
In order to prove the lower bound in (3.51) we will need the following.
Proposition 3.9.
For and ,
Proof.
We will use a technique similar to [12, Theorem 7.34]. According to (3.42),
On the other hand, from (see [18, Lemma 2.1]), if and , we have
Thus, for and large enough, (3.57) follows.
Finally, from (3.49), we obtain
The proof of Proposition 3.9 is complete.
From (3.57), for and ,
Thus, using (3.56) and (3.61), the statement follows.
4. Necessary Conditions for the Norm Convergence
The analysis of the norm convergence of partial sums of the Fourier expansions in terms of Jacobi polynomials has been done by many authors. See, for instance, [19–21], and the references therein.
Let be the JacobiSobolev orthonormal polynomials, that is,
For , its Fourier expansion in terms of JacobiSobolev orthonormal polynomials is
where
Let be the th partial sum of the expansion (4.2)
Theorem 4.1.
Let , and . If there exists a constant such that
for every , then with
Proof.
For the proof, we apply the same argument as in [20]. Assume that (4.5) holds. Then,
Consider the linear functionals
on . Hence, for every in holds. From the BanachSteinhaus theorem, this yields . On the other hand, by duality (see, for instance, [1, Theorem 3.8]), we have
where is the conjugate of . Therefore,
On the other hand, from (3.51), we obtain the Sobolev norms of JacobiSobolev orthonormal polynomials
for and . Now, from (4.11), it follows that the inequality (4.10) holds if and only if .
The proof of Theorem 4.1 is complete.
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Acknowledgments
The authors thank the referees for the careful revision of the manuscript. Their comments and suggestions have contributed to improve substantially its presentation. The work of F. Marcellán has been supported by Dirección General de Investigación, Ministerio de Ciencia e Innovación of Spain, Grant no. MTM200912740C0301.
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Keywords
 Compact Subset
 Orthogonal Polynomial
 Fourier Expansion
 Jacobi Polynomial
 Sobolev Norm