1. Introduction and Preliminaries
The concept of the Mehler-Fock transform has its roots in the groundbreaking contributions of F. G. Mehler [
5] and V. A. Fock [
2]. Evolving from their seminal work, it emerged as a distinct integral transform, finding applications across a broad spectrum of mathematical physics problems. Extensive research by various scholars has led into its properties and applications. For an in-depth exploration, interested readers can consult the following references, among others [
3,
4,
7,
8,
9,
10,
11,
13].
We consider the generalized Mehler-Fock transform of a suitable complex-valued function
f on
analyzed by B. L. J. Braaksma and B. M. Meulenbeld [
11] and R. S. Pathak [
13] (chapter 11) given by
where
are complex numbers with
and
is the associated Legendre function of the first kind [
1] (Chapter 3) defined by
where
is the Gauss hypergeometric function [
1] (p. 57).
A corresponding inversion formula of (
1) for suitable
f is given by
where
The conditions of validity of (
1) and (
2) are provided by Braaksma and Meulenbeld [
11].
Notice that for
one obtains the Mehler-Fock transform of zero order [
12] (Section 7-6, p. 390) and [
11] (Theorem 7, p. 247)
and a corresponding inversion formula given by
The case
,
is contemplated in [
11] (Theorem 7, p. 247)
and a corresponding inversion formula given by
From [
13] (formula (11.1.8), p. 345)
From [
13] (formula (11.1.9), p. 345)
From [
13] (formula (11.1.9), p. 345)
where
C is a constant independent of
x and
,
,
,
.
Now, observe that from () the convergence of the integral
is not assured in
∞.
For
we consider the vector space
consisting of all complex-valued measurable functions
f on
such that
. In their recent study [
6], Maan and Negrín made extensive use of spaces of type
.
A norm
on
is given by
With this norm, the map
where for any
:
is an isometric isomorphism from
to
. Thus, since
is complete, then the space
becomes a Banach space.
The , denotes as it is usual the space of compactly supported functions on which are k-times differentiable with continuity.
The content of this paper is as follows:
Section 1 is concerned with the definitions and useful results which are used in the entire sequel.
Section 2 provides an inversion formula for the generalized Mehler-Fock transform given by (
1).
Section 3 deals with the continuity features over Lebesgue spaces and Parseval-Goldstein type relations for the generalized Mehler-Fock transform given by (
1).
Section 4 gives concluding remarks.
2. An Inversion Formula over the Spaces
By means of [
11] (Theorem 5, p. 245) one obtains the next inversion formula for the transform (
1) over the spaces
,
.
Theorem 1 (Inversion formula).
Assume , , and let be complex numbers with and . Suppose that the function f is continuous at the point x and of bounded variation in the neighbourhood of the point x. Then
where is given by (3).
Proof. Observe that being
,
, the function
, satisfies the condition (0.6) of p. 236 of [
11].
Also the function g is continuous at the point and of bounded variation in the neighbourhood of the point .
Thus from Theorem 5 of [
11] one has
Now, taking in (
13)
, and taking into account that
, one obtains
□
As a consequence of Theorem 1 one obtains the next result.
Corollary 2 (Injectivity) Assume that the functions , , and let be complex numbers with and . Suppose that the function f and g are continuous on and of bounded variation in the neighbourhood of each point of . Then, if on it follows that on .
3. Boundedness Properties and Parseval-Goldstein Type Relations Over the Spaces
Observe that for
being complex numbers with
,
,
,
, one obtains from (
12)
Now, from (
10) and () the integral
converges for
.
Thus the next result holds.
Proposition 3. Set
be complex numbers with
. The generalized Mehler-Fock transform
given by (
1) is a bounded linear operator from
,
, into
. If
,
, then
and
is a continuous function on
. Moreover, the generalized Mehler-Fock transform
is a continuous map from
,
, to the Banach space of bounded continuous functions on
.
Proof. Let
be arbitrary. Since the map
is continuous for each fixed
, we have
Further, we have that
is dominated by the integrable function
Therefore, by using dominated convergence theorem, we get
which tends to 0 as
.
Thus, is a continuous function on .
Since for each
and from (
14) one has
then
is a bounded function.
The linearity of the integral operator implies that the
transform is linear. Also from (
15) we get that
and hence
is a continuous linear map. □
Also, the next result holds.
Proposition 4. Set
be complex numbers with
,
,
w be a measurable function on
such that
a.e. on
and
. Then, for
,
is a bounded linear operator.
Example 1.
Examples of weights w for Proposition 4 are:
For a suitable function
g we denote
where
m and
n are complex numbers with
.
The exists for .
In fact, from (
12)
for each
.
Now, observe that for
and having into account (
10) and () the expression
Thus, for
one has
So one obtains the next result.
Proposition 5. Set
be complex numbers with
. The linear operator
given by (
16) is a bounded linear operator from
into
,
.
The next result holds.
Proposition 6. Set
be complex numbers with
and
w be a measurable function on
such that
a.e. on
and
. If
. Then
is a bounded linear operator.
Proof. From (
17) one has
for some
, and so the result holds. □
Example 2.
Examples of weights w for Proposition 6 are:
The next Parseval-Goldstein type relation holds.
Theorem 7.
Set be complex numbers with , , and , then the following Parseval-Goldstein type relation relation holds
Proof. Applying Fubini’s theorem in the following we get
Observe that from (
17) the
exists for
. □
Consider the differential operator
From [
13]
and then for
From (
20) and
,
, it follows
Then from Theorem 7 one has the next result.
Theorem 8.
If , , be complex numbers with , and , then the following Parseval-Goldstein type relation holds
4. Conclusions
In conclusion, this paper has been dedicated to the establishment of boundedness properties and Parseval-Goldstein type relations for the generalized Mehler-Fock transform, as first introduced by B. L. J. Braaksma and B. Meulenbeld in 1967 and subsequently by R. S. Pathak in 1997. Moreover, we have derived an inversion formula for this transform across Lebesgue spaces. These results not only contribute to a deeper understanding of the generalized Mehler-Fock transform but also pave the way for future investigations into analogous properties within the spaces of type for diverse integral transforms.
Declarations Note
The manuscript has no associated data.
Conflicts of Interest
No potential conflict of interest was reported by the authors.
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