Mathematical Problems in Engineering

Volume 2013, Article ID 217656, 7 pages

http://dx.doi.org/10.1155/2013/217656

## On Multiple Convolutions and Time Scales

^{1}Mathematics Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia^{2}Department of Mathematics and Institute for Mathematical Research, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia^{3}Department of Mathematics, University of Leicester, Leicester LE1 7RH, UK

Received 23 September 2013; Accepted 7 November 2013

Academic Editor: Suares Clovis Oukouomi Noutchie

Copyright © 2013 Hassan Eltayeb et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The properties of the multiple Laplace transform and convolutions on a time scale are studied. Further, some related results are also obtained by utilizing the double Laplace transform. We also provide an example in order to illustrate the main result.

#### 1. Introduction

A time scale is a nonempty closed subset of the real numbers and it has the topology that it inherits from the real numbers with the standard topology. It is also well known that if the time scale is the set of real numbers, then the dynamic equation is a differential equation, whilst if the time scale is the set of integers, then the dynamic equation is reduced to a difference equation. Thus integral transform of differential and integral calculus on time scales allows us to develop a theory of differential equations. For the single Laplace transform on time scales, see [1–3]. Similarly, in [4–8], the authors defined the time scale to be any nonempty closed subset of the real numbers and provided the motivation and formulation of delta derivatives on a time scale as well as the properties of delta derivatives and integrals. In [2], Bohner and Peterson defined the Laplace transform of a time scale for function of single variable as follows.

*Definition 1. *For , the time scale or generalized Laplace transform of , denoted by or , is given by
where consists of all for which the improper integral exists and for all .

*Definition 2. *The function is said to be of exponential type I if there exist constants such that . Furthermore, is said to be of exponential type II if there exist constants , such that .

The following two theorems were proved in [3].

Theorem 3. *The integral converges absolutely for if is of exponential type II with exponential constant . For more details, see [3].*

Theorem 4. *Let be regressive. Then**
(I)**
provided that ,**
(II)**
provided that ,**
(III)**
provided that .*

The following theorem was also studied by Bohner and Peterson in [2].

Theorem 5. *If is such that is regulated, then
**
for all such that .*

Throughout the study, the following notation will be useful, where are time scales and

In [2], further it was proved that was satisfied, where , by using the property

Later in [9], Bohner and Guseinov defined the single convolution operation on time scale of two functions by the following formula:

In this study, we introduce double Laplace transform on time scales and study some of the properties in solving partial differential equations. Further, we also extend the -Laplace transform, which is given by Jackson in [10], to the multiple -Laplace transform as follows.

*Definition 6. *Let , , and . The -double Laplace transform of the function (for ) with respect to is given by
provided that the integral exists, further , and , for all .

Lemma 7. *If are regressive, then
*

*Proof. *By using the above analysis, we have

As an example, we can easily compute , where are constants such that .

We have For and with and , we can easily see that and then the double Laplace transforms are given by

Similarly, we have

Proposition 8. *Suppose , are two time scales such that and for some and all . Let and
**
Then
*

*Proof. *Since
and on using the following relation:
we have
Since and , we get
Thus we obtain that
and it follows that

Theorem 9. *Let be regulated and let
**
for . Then
**
for .*

*Proof. *By using the definition of the double Laplace transform on a time scale, we have
and on using (21), we have
Now, applying integration by parts, and on using the fact that together with the fundamental theorem of calculus, we obtain that

*Example 10. *Let , , , and for and , and whilst for , and , , where and the symbol denotes the tensor product. Then we have

Now assume that such that is continuous. Then the -double Laplace transform holds for those regressive with respect to and which further satisfies We next extend the result that was proved by Davis et al. in [3].

Proposition 11. *Let , . Assume that is one of the following functions:
**
If are regressive and satisfy
**
then
*

*Proof. *Let us study the case . By using (21), we get
and by using Theorem 4 and (21) the right hand side of equation (37) is
Then

Theorem 12. *Let and suppose that and are continuous for all and , respectively; then the -double Laplace transforms for and are given by
**
for all with
**
respectively.*

*Proof. *Let . On using the definition of the -single Laplace transform and integrating parts of the function , we have
Then the single Laplace transform with respect to is given by
Similarly, Laplace transform for (44) with respect to is given by
Then (45) is called the -double Laplace transform for the function . Similarly, if in (41), the -double Laplace transform for the function is given by
Now, in order to obtain the -double Laplace transform for the function
with respect to , we proceed as follows: first of all by taking the -single Laplace transform with respect to the variable and on using the fact that
we have
Thus on using (44), we obtain that
In a similar way, the single Laplace transform with respect to is given by
Further, the -double Laplace transform for the function with respect to is given by
Finally, note that we can generalize the proof for .

Next, we define the double convolution of two time scale functions as follows.

*Definition 13. *The double convolution of the function , is given by

In the next theorem we prove the properties of the double convolution on a time scale as follows.

Theorem 14. *Let , be integrable functions on a time scale. Then the following properties are satisfied:
*

*Proof. *We can easily show that convolution is commutative by using the definition of the double convolution
For the proof of the associative law, we also use the definition as follows:

Theorem 15 (convolution theorem). *Let , be regulated functions. Then the double Laplace transform of the double convolution is given by
*

*Proof. *By using the definition of the double Laplace transform on a time scale, we obtain that
where and the symbol denotes the tensor product. Thus we have

Finally, we give the solution to the wave equation in one dimension as follows. Consider the wave equation in the form of under the conditions Then by formally taking the double Laplace transform, we get By using partial fractions, we have Thus the solution to the above equation is given by In particular, if we consider the case , , for all , then for any constant and so the solution is given by

#### Conflict of Interests

The authors declare that they have no conflict of interests regarding the publication of paper.

#### Acknowledgment

The authors are grateful to the referees for the useful comments regarding presentation in the early version of the paper. The authors also gratefully acknowledge that this project was supported by King Saud University, Deanship of Scientific Research College of Science Research Center.

#### References

- H. A. Agwa, F. M. Ali, and A. Kılıçman, “A new integral transform on time scales and its applications,”
*Advances in Difference Equations*, vol. 2012, article 60, 2012. View at Publisher · View at Google Scholar - M. Bohner and A. Peterson, “Laplace transform and Z-transform: unification and extension,”
*Methods and Applications of Analysis*, vol. 9, no. 1, pp. 155–162, 2002. View at Google Scholar - J. M. Davis, I. A. Gravagne, B. J. Jackson, R. J. Marks, and A. A. Ramos, “The Laplace transform on time scales revisited,”
*Journal of Mathematical Analysis and Applications*, vol. 332, no. 2, pp. 1291–1306, 2007. View at Publisher · View at Google Scholar · View at Scopus - R. P. Agarwal, C. Ahlbrandt, M. Bohner, and A. Peterson, “Discrete linear Hamiltonian systems: a survey,”
*Dynamic Systems and Applications*, vol. 8, pp. 307–333, 1999. View at Google Scholar - R. P. Agarwal and M. Bohner, “Quadratic functionals for second order matrix equations on time scales,”
*Nonlinear Analysis: Theory, Methods and Applications*, vol. 33, no. 7, pp. 675–692, 1998. View at Google Scholar · View at Scopus - R. P. Agarwal and M. Bohner, “Basic calculus on time scales and some of its applications,”
*Results in Mathematics*, vol. 35, pp. 3–22, 1999. View at Google Scholar - C. D. Ahlbrandt, M. Bohner, and J. Ridenhour, “Hamiltonian systems on time scales,”
*Journal of Mathematical Analysis and Applications*, vol. 250, no. 2, pp. 561–578, 2000. View at Publisher · View at Google Scholar · View at Scopus - L. Erbe and S. Hilger, “Sturmian theory on measure chains,”
*Differential Equations and Dynamical Systems*, vol. 1, pp. 223–246, 1993. View at Google Scholar - M. Bohner and G. S. Guseinov, “The convolution on time scales,”
*Abstract and Applied Analysis*, vol. 2007, Article ID 58373, 24 pages, 2007. View at Publisher · View at Google Scholar - B. Jackson, “Partial dynamic equations on time scales,”
*Journal of Computational and Applied Mathematics*, vol. 186, no. 2, pp. 391–415, 2006. View at Publisher · View at Google Scholar · View at Scopus