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Characterization of the support for the hypergeometric Fourier transform of the W-invariant functions and distributions on and Roe’s theorem
Journal of Inequalities and Applications volume 2014, Article number: 99 (2014)
Abstract
In this paper, we establish real Paley-Wiener theorems for the hypergeometric Fourier transform on . More precisely, we characterize the functions of the generalized Schwartz space and of , , whose hypergeometric Fourier transform has bounded, unbounded, convex, and nonconvex support. Finally we study the spectral problem on the generalized tempered distributions .
MSC:35L05, 22E30.
1 Introduction
We consider the differential-difference operators , , associated with a root system ℛ and a multiplicity function k, introduced by Cherednik in [1], called Cherednik operators in the literature. These operators were helpful for the extension and simplification of the theory of Heckman-Opdam, which is a generalization of the harmonic analysis on the symmetric spaces (cf. [2–4]).
The Paley-Wiener theorems for functions and distributions are most useful theorems in harmonic analysis. These theorems have as aim to characterize functions with compact support through the properties of the analytic extensions of their classical Fourier transform on . Recently there has been a great interest to real Paley-Wiener theorems.
The first theorem given by Bang (cf. [5]) can be stated as follows. Let f be a -function on ℝ such that for all , and let the function belong to the Lebesgue space , then the limit exists and we have
where is the classical Fourier transform of f. Next the analogue of this theorem was established for many other integral transforms (cf. [6–9]).
Motivated by the treatment in the Euclidean setting, we will derive in this paper new real Paley-Wiener theorems for the hypergeometric Fourier transform, on some Lebesgue space and on a generalized tempered distribution space .
The remaining part of the paper is organized as follows. In Section 2 we recall the main results as regards the harmonic analysis associated with the Cherednik operators and the Heckman-Opdam theory (cf. [1–3]). Section 3 is devoted to a study of the -Schwartz functions such that the supports of their hypergeometric Fourier transform are compact. Next we prove a new real Paley-Wiener theorem for the hypergeometric Fourier transform on generalized Paley-Wiener spaces. In Section 4 we characterize the functions in the generalized Schwartz spaces such that their hypergeometric Fourier transform vanishes outside a polynomial domain. We give also a necessary and sufficient condition for functions in such that their hypergeometric Fourier transform vanishing in a neighborhood of the origin. In Section 5 we study the generalized tempered distributions with spectral gaps. Finally, in the last section we prove Roe’s theorem for the hypergeometric Fourier transform.
2 Preliminaries
This section gives an introduction to the theory of Cherednik operators, hypergeometric Fourier transform, and hypergeometric convolution. Our main references are [1–4, 10].
2.1 Reflection groups, root systems, and multiplicity functions
The basic ingredients in the theory of Cherednik operators are root systems and finite reflection groups, acting on with the standard Euclidean scalar product and . On , denotes also the standard Hermitian norm, while .
For , let be the coroot associated to α and let
be the reflection in the hyperplane orthogonal to α.
A finite set is called a root system if . and for all . For a given root system ℛ the reflections , generate a finite group , called the reflection group associated with ℛ. All reflections in W correspond to suitable pairs of roots. We fix a positive root system for some .
Let
be the positive chamber. We denote by its closure.
A function is called a multiplicity function if it is invariant under the action of the associated reflection group W. For abbreviation, we introduce the index
Moreover, let denote the weight function
We note that this function is W invariant and satisfies
where
2.2 The eigenfunctions of the Cherednik operators
The Cherednik operators , , on associated with the finite reflection group W and multiplicity function k are given by
The operators can also be written in the form
with
In the case , for all , the , , reduce to the corresponding partial derivatives.
Example 1 For , the root systems are , or with α the positive root. We take the normalization .
For , we have the Cherednik operator
with . This operator can also be written in the form
For , we have the Cherednik operator
This operator can also be written in the form
with .
For , we have the Cherednik operator
with . It is also equal to
The operators (2.6), (2.7), and (2.8) are particular cases of the differential-difference operator
which is refereed to as the Jacobi-Cherednik operator (cf. [11, 12]).
The Heckman-Opdam Laplacian is defined by
where △ and ∇ are, respectively, the Laplacian and the gradient on .
The Heckman-Opdam Laplacian on W-invariant functions is denoted by and we have the expression
Example 2 For , and , , the Heckman-Opdam Laplacian is the Jacobi operator defined for even functions f of class on ℝ by
with .
We denote by the eigenfunction of the operators , . It is the unique analytic function on which satisfies the differential-difference system
It is called the Opdam-Cherednik kernel.
We consider the function defined by
This function is the unique analytic W-invariant function on , which satisfies the differential equations
for all W-invariant polynomial p on and .
In particular for all we have
The function is called the Heckman-Opdam kernel.
The functions and possess the following properties.
-
(i)
For all , the functions and are entire on .
-
(ii)
We have
and
-
(iii)
There exists a positive constant such that
(2.11)
and
-
(iv)
We have
-
(v)
Let p and q be polynomials of degree m and n. Then there exists a positive constant such that for all and for all , we have
(2.12) -
(vi)
The preceding estimate holds true for too.
Example 3 When and , and , , the Opdam-Cherednik kernel is given for all and by
where is the Jacobi function of index defined by
with and is the Gauss hypergeometric function.
In this case the Heckman-Opdam kernel is given for all and by
2.3 The hypergeometric Fourier transform on W-invariant function and distribution spaces
We denote by
the space of -functions on , which are W-invariant;
the space of -functions on , which are W-invariant and with compact support;
the Schwartz space of rapidly decreasing functions on ;
the space of -functions on which are W-invariant, and such that for all , we have
where
the space of entire functions on , which are W-invariant, rapidly decreasing and of exponential type;
the space of entire functions on , which are W-invariant, slowly increasing and of exponential type;
the space of distributions on , which are W-invariant;
the space of distributions on which are W-invariant and with compact support;
the space of tempered distributions on , which are W-invariant. It is the topological dual of ;
the topological dual of . We have
with c a normalizing constant and if .
The measure is called the symmetric Plancherel measure or the Harish-Chandra measure (cf. [2, 4]).
Remark 1 The function is positive, continuous on , and it satisfies the estimate
for some .
, , is the space of measurable functions f on which are W-invariant and satisfy
, , is the space of measurable functions f on which are W-invariant and satisfy
The hypergeometric Fourier transform of a function f in is given by
Proposition 1 The transform is a topological isomorphism from
-
(i)
onto ,
-
(ii)
onto .
The inverse transform is given by
Proposition 2 For f in the function is continuous on and we have
where is the constant given by the relation (2.11).
Proposition 3 (i) Plancherel formula. For all f, g in (resp. ) we have
-
(ii)
Plancherel theorem. The transform extends uniquely to an isomorphism from onto .
Proposition 4 For all f in such that belongs to , we have the inversion formula
2.4 The hypergeometric convolution
Definition 1 Let y be in . The hypergeometric translation operator is defined on by
Using the hypergeometric translation operator, we define the hypergeometric convolution product of functions as follows.
Definition 2 The hypergeometric convolution product of f and g in is the function defined by
Proposition 5 ([10])
-
(i)
Let . Then
(2.18)
and
-
(ii)
Let f be in and g in . Then
(2.20)
Definition 3 (i) We define the hypergeometric Fourier transform of a distribution S in by
-
(ii)
The hypergeometric Fourier transform of a distribution S in is defined by
Theorem 1 The transform is a topological isomorphism from
-
(i)
onto ,
-
(ii)
onto .
Let τ be in . We define the distribution , by
This distribution satisfies the following property:
3 Functions with compact spectrum
We consider f in . We define the distribution in by
Notations We denote by
the space of functions in with compact support;
-
(a)
for all ;
-
(b)
.
the space of entire functions f on of exponential type such that belongs to .
Theorem 2 The hypergeometric Fourier transform is bijective from onto .
Proof (i) We consider the function f on given by
with .
By derivation under the integral sign and by using the inequality (2.12), we deduce that the function f is entire on and of exponential type. On the other hand the relation (3.1) can also be written in the form
Thus from Proposition 3 the function belongs to . Thus .
-
(ii)
Reciprocally let ψ be in . From [10] there exists with support in the boule , such that
(3.2)
On the other hand as belongs to , from Proposition 3 there exists h in such that
Thus from (3.2), for all we have
Thus using (2.14) we deduce that
On the other hand (3.3) implies
But from Proposition 3 we deduce that
Thus the previous relation and the relation (3.4) imply
This relation shows that the support of h is compact. Then . □
In the following will be denoted by f.
Definition 4 (i) We define the support of and we denote it by suppg, the smallest closed set, outside of which the function g vanishes almost everywhere.
-
(ii)
We denote by
the radius of the support of g.
Remark 2 It is clear that is finite if and only if g has compact support.
Proposition 6 Let such that for all , the function belongs to . Then
Proof We suppose that , otherwise and (3.5) is trivial.
Assume now that g has compact support with . Then
Thus we deduce that
On the other hand, for any positive ε we have
Hence
Thus
We prove now the assertion in the case where g has unbounded support. Indeed for any positive N, we have
Thus
This implies that
□
Notations We denote by
the space of functions in with compact support;
, for ;
, for .
Definition 5 We define the generalized Paley-Wiener spaces and as follows.
-
(i)
is the space of functions satisfying
-
(ii)
.
The real -Paley-Wiener theorem for the hypergeometric Fourier transform can be formulated as follows.
Theorem 3 The hypergeometric Fourier transform is a bijection
-
(i)
from onto ;
-
(ii)
from onto .
Proof Let . Using (2.22), we see that the function
On the other hand from Proposition 3 we deduce that
Thus using Proposition 6 we conclude that has compact support with
Conversely let . Then for any , and belongs to . On the other hand from Proposition 3 we have
Thus .
-
(ii)
We deduce the result from (i). □
We finish this section with an application on the generalized Schrödinger equation, which was introduced and studied in [13].
Corollary 1 Let . Then f belongs to if and only if the solution of the Cauchy problem for the generalized Schrödinger equation
has the following properties:
-
(i)
as a function of t, it has an analytic extension , to the complex plane as an entire function,
-
(ii)
it has exponential type σ in the variable z, that is,
and it is bounded on the real line.
4 Hypergeometric Fourier transform of functions with polynomial domain support
Notation We denote by the set of polynomials on ℝ with complex coefficients.
Definition 6 Let u be a distribution on and P a polynomial. Then we let
where by convention if .
Theorem 4 Let . For any function the following relation holds:
Proof We consider in . The proof is divided in two steps. In the following step we suppose that
First step: In this step we shall prove that
In this case we assume firstly that has compact support. Hölder’s inequality gives
for . Using the relation (2.4), we obtain
Consequently for all , we deduce that
Using the continuity of we can show that
with positive constants C and integers M, m, independent of n. Using Leibniz’s rule we deduce that
with C is a constant independent of n. Hence, from the previous inequalities we obtain
In particular, if , the identity (4.1) follows at once.
Second step: In this step we shall prove that
Fix . We can assume that . We will show that
for any fixed such that .
To this end, choose and fix such that , and
For , let . In the following we want to estimate . Indeed as above we have
with . Using the continuity of we can show that
with positive constants C and integers M, m, independent of n. Using Leibniz’s rule we deduce that
Then
Hence, from the Hölder inequality we obtain
Since , we deduce that
Thus
In particular, if , the identity (4.1) follows at once.
Hence the proof of the theorem is finished. □
Definition 7 Let P be in . We define the domain by
We have the following result.
Corollary 2 Let . The hypergeometric Fourier transform vanishes outside a polynomial domain , if and only if
Remark 3 If we take , then , and Theorem 4 and Corollary 2 characterize functions such that the support of their hypergeometric Fourier transform is .
Theorem 5 Let . Let f be in , for some , such that for all , the function belongs to . Then
Proof Let now f be in , for some such that for all , the function belongs to and has compact support.
Let . We choose such that on a neighborhood of and for all . Moreover it is easy to see that , hence from (2.18) we deduce
Thus from Theorem 4
Since the case is trivially true, we obtain
Now we consider f in such that the function belongs to and we proceed as in Theorem 4, step 2 to obtain
and the theorem follows. □
The following theorem gives the radius of the large disc on which the hypergeometric Fourier transform of functions in vanishes everywhere.
Theorem 6 Let be non-negligible. We consider the sequence
where
Then
where
Proof To prove (4.8) it is sufficient to verify the equivalent identity
Using (2.20) we deduce that the hypergeometric Fourier transform of is . Then by applying Proposition 3 we obtain
On the other hand it is well known that if m is the Lebesgue measure on and U a subset of such that , then for all ϕ in the Lebesgue space , , we have
By applying formula (4.12) with
and using the fact that , we obtain
This is the relation (4.10). □
A function is the hypergeometric Fourier transform of a function vanishing in a neighborhood of the origin, if and only if, , or equivalently, if and only if the limit (4.10) is less than 1. Thus we have proved the following result.
Corollary 3 The condition
is necessary and sufficient for a function to have its hypergeometric Fourier transform vanishing in a neighborhood of the origin.
Remark 4 From Theorem 3 and Corollary 3 it follows that the support of the hypergeometric Fourier transform of a function in is in the torus , if and only if,
Theorem 7 For any function the following relation holds:
In particular, a function is the inverse hypergeometric Fourier transform of a function in vanishing in , if and only if we have
Proof A similar proof to that of Theorem 4 gives the result. □
5 Real Paley-Wiener theorems for the hypergeometric Fourier transform on
Theorem 8 Let , and suppose the set is compact for a polynomial and a constant . Then the support of is contained in , if and only if for each , there exist and a positive constant such that
for all and .
Proof Assume that the support of is contained in the compact . We choose such that on an open neighborhood of support of , and outside . As is of order N, there exists a positive constant C such that for all
Thus from the Leibniz formula, relation (2.5), we deduce the result.
Conversely we assume that we have (5.1).
Suppose is fixed and such that , for some . Choose and fix such that , and put . We have
Hence, from the Hölder inequality we obtain
We proceed as in Theorem 4, step 2, and we prove that
Thus
Thus we deduce , which implies that . □
Corollary 4 Let such that is compact. Let . Then
where is defined as the infimum of all for which there exist N and , such that for all and
Remark 5 We note that the analogue of Theorem 8 and Corollary 4, studied in [14, 15], is missing it the term (as a typing error).
Notations Let . We denote by
where and .
Theorem 9 Let . Then the support of is included in , with , if and only if for all we have
Proof Let and such that
Let satisfying . We have to prove that
Let satisfying for all and . Then for all the function is in and we can write
and by (2.22), we have
The hypothesis implies that in . Moreover from the Leibniz formula we deduce that in . So using the Banach-Steinhaus theorem we prove that
Conversely, let and such that . We are going to prove that for all
Let and choose and satisfying on a neighborhood of and for all . Then for all we have
and then
Finally we deduce the result by using the fact that in . □
From the previous theorem we obtain the following.
Corollary 5 We have
Theorem 10 Let such that for all . Let . Then the support of is included in , , if and only if for all we have
Proof Let and such that
Let satisfying . We have to prove that
Let satisfying and . Then for all the function is in and we can write
and by (2.22), we have
The hypothesis implies that in . Moreover, from the Leibniz formula we deduce that in . So applying the Banach-Steinhaus theorem we prove that
Conversely, let and such that . We are going to prove that for all
Let and choose and satisfying for and for all . Then for all we have
and then
Finally we deduce the result by using the fact that in . □
From the previous theorem we obtain the following.
Corollary 6 We have
6 Roe’s theorem associated with the Heckman-Opdam Laplacian operator
In [16] Roe proved that if a doubly infinite sequence of functions on ℝ satisfies and for all and , then where a and b are real constants. This result was extended to by Strichartz [17] where is substituted for by the Laplacian on as follows.
Theorem (Strichartz)
Let be a doubly infinite sequence of measurable functions on such that for all , (i) for some constant and (ii) for some , . Then .
The purpose of this section is to generalize this theorem for the Heckman-Opdam Laplacian operator. We now state our main result.
Theorem 11 Suppose is real-valued. Let and let be a sequence of W-invariant complex-valued functions on so that
and
where satisfies the sublinear growth condition,
Then where and . If 1 (or −1) is not in the range of P then (or ).
We break the proof up into three steps. In the first step we consider the hypergeometric Fourier transform of , which exists as a distribution.
Lemma 1 Let . Let is a sequence of W-invariant functions on satisfying
and
for all , then
Proof First we show that is supported in . To do this we need to show that if and . Since is compact, there is some so that , for all . Then
Choose an integer m with . A calculation, using the hypothesis of the lemma and the Cauchy-Schwartz inequality, implies
Using the continuity of and the fact that ϕ is supported in for some fixed , it is not hard to prove that the right-hand side of this goes to zero as and so . To complete the proof we need to show that is also supported in , which means if ϕ is supported in . Here we use (6.3) to obtain
and the argument proceeds as before. □
In the next step in the proof we assume firstly that −1 is not a value of , and we show that .
Lemma 2 There exists an integer N such that
Proof From the growth conditions on the sequence , Lemma 1, and the assumption that , we obtain
As is a continuous linear functional on , there is a constant C and there are integers m and N so that
for all when the topology on the space is defined by the seminorms
Thus the distribution is of order ≤N. For this N we want to prove that
To simplify the notation set . Then we need to show, for any compactly supported function ϕ, that
Let be a function with on and outside .
Set , . Then in a neighborhood of
Thus by (6.7) we have
We proceed as [18] to prove that as . Thus (6.6) is proved.
Inverting the hypergeometric Fourier transform in (6.6) yields
This equation implies
We shall now show that we can take in (6.9). If not then . Let p be the largest positive integer so that . Clearly . Thus
will satisfy
Write
for constants . Then
If
then this and (6.1) imply
By (6.2) these satisfy the sublinear growth condition,
An induction using (6.10) implies for that
Thus
Letting and using (6.12) implies . But this contradicts (6.10). Consequently, in (6.9). This completes the proof in the case that −1 is not in the range of P.
In the case that 1 is not in the range of P we apply the same argument to to conclude .
In the general case, let . Then . We have and . Thus we can (as before) conclude for the sequence that
Set and . Then , and . This completes the proof of Theorem 11. □
Remark 6 (i) If we take , then and Theorem 11 gives . This characterizes the eigenfunctions f of the generalized Laplace operator with polynomial growth in terms of the size of the powers , .
-
(ii)
We note that the analogue of Theorem 11 and Lemma 1, studied in [14, 15], is missing it the term (as a typing error).
-
(iii)
We note that Barhoumi and Mili in [19] have studied the range of the generalized Fourier transform associated with a Cherednick type operator on the real line, and have generalized the Roe’s theorem for this operator.
As an application of the above theorem we have the following.
Theorem 12 If, in Theorem 11, we replace (6.2) with
for all , then the span of is finite dimensional. Moreover, , where, for some integer N, and . Thus (or ) is a generalized eigenfunction of with eigenvalue 1 (or −1).
Before we demonstrate this theorem we needs the following lemma.
Lemma 3 ([18])
Let X be a finite dimensional complex vector space, and let be a linear map with eigenvalues . Then , where and .
Proof We prove Theorem 12.
We first prove the result under the assumption that . Using the growth condition (6.13) and Lemma 3, we may still conclude that
But then, as before, we can conclude that (6.9) holds. But this is enough to complete the proof in this case. A similar argument shows that if , then .
In the general case we again let and . Then and the span of is finite dimensional. The map takes the span of onto the span of . Thus X is finite dimensional. Any will have inside the set defined by . From this the only possible eigenvalues of restricted to X are 1 and −1, as it is not hard to show. The result now follows from the last lemma. □
References
Cherednik I: A unification of Knizhnik-Zamolodchnikov equations and Dunkl operators via affine Hecke algebras. Invent. Math. 1991, 106: 411-432. 10.1007/BF01243918
Opdam EM: Harmonic analysis for certain representations of graded Hecke algebras. Acta Math. 1995, 175: 75-121. 10.1007/BF02392487
Opdam EM Mem. Math. Soc. Japon 8. Lecture Notes on Dunkl Operators for Real and Complex Reflection Groups 2000.
Schapira B: Contributions to the hypergeometric function theory of Heckman and Opdam: sharp estimates, Schwartz spaces, heat kernel. Geom. Funct. Anal. 2008,18(1):222-250. 10.1007/s00039-008-0658-7
Bang HH: A property of infinitely differentiable functions. Proc. Am. Math. Soc. 1990,108(1):73-76. 10.1090/S0002-9939-1990-1024259-9
Andersen NB: On the range of the Chébli-Trimèche transform. Monatshefte Math. 2005, 144: 193-201. 10.1007/s00605-004-0256-1
Chettaoui C, Othmani Y, Trimèche K:On the range of the Dunkl transform on . Anal. Appl. 2004,2(3):177-192. 10.1142/S0219530504000370
Mejjaoli H, Trimèche K:Spectrum of functions for the Dunkl transform on . Fract. Calc. Appl. Anal. 2007,10(1):19-38.
Tuan VK: Paley-Wiener transforms for a class of integral transforms. J. Math. Anal. Appl. 2002, 266: 200-226. 10.1006/jmaa.2001.7740
Trimèche, K: Hypergeometric convolution structure on -spaces and applications for the Heckman-Opdam theory. Prébublication of Faculty of Sciences of Tunis
Anker J-PH, Ayadi F, Sifi M: Opdam’s hypergeometric functions: product formula and convolution structure in dimension 1. Adv. Pure Appl. Math. 2012,3(1):11-44.
Gallardo L, Trimèche K: Positivity of the Jacobi-Cherednik intertwining operator and its dual. Adv. Pure Appl. Math. 2010,1(2):163-194.
Mejjaoli H: Cherednik-Sobolev spaces and applications. Afr. J. Math. 2013. 10.1007/s13370-013-0191-1
Mejjaoli H: Spectrum of functions and distributions for the Jacobi-Dunkl transform on ℝ. Mediterr. J. Math. 2013,10(2):753-778. 10.1007/s00009-012-0206-4
Mejjaoli H: Spectral theorems associated with the Jacobi-Cherednik operator. Bull. Sci. Math. 2013. 10.1016/j.bulsci.2013.10.004
Roe J: A characterization of the sine function. Math. Proc. Camb. Philos. Soc. 1980, 87: 69-73. 10.1017/S030500410005653X
Strichartz RS: Characterization of eigenfunctions of the Laplacian by boundedness conditions. Trans. Am. Math. Soc. 1993, 338: 971-979. 10.1090/S0002-9947-1993-1108614-1
Howard R, Reese M: Characterization of eigenfunctions by boundedness conditions. Can. Math. Bull. 1992, 35: 204-213. 10.4153/CMB-1992-029-x
Barhoumi N, Mili M: On the range of the generalized Fourier transform associated with a Cherednick type operator on the real line. Arab J. Math. Sci. 2013. 10.1016/j.ajmsc.2013.11.001
Acknowledgements
The first author gratefully acknowledges the Deanship of Scientific Research at the University of Taibah University on material and moral support in the financing of this research project No. 6054. The authors are deeply indebted to the referees for providing constructive comments and helps in improving the contents of this article. The first author thanks NB Andersen for his help and encouragement.
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Mejjaoli, H., Trimèche, K. Characterization of the support for the hypergeometric Fourier transform of the W-invariant functions and distributions on and Roe’s theorem. J Inequal Appl 2014, 99 (2014). https://doi.org/10.1186/1029-242X-2014-99
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DOI: https://doi.org/10.1186/1029-242X-2014-99