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Singular integral equation involving a multivariable analog of Mittag-Leffler function
Advances in Difference Equations volume 2014, Article number: 252 (2014)
Motivated by the recent work of the second author (Özarslan in Appl. Math. Comput. 229:350-358, 2014), we present, in this paper, some fractional calculus formulas for a mild generalization of the multivariable Mittag-Leffler function, a Schläfli’s type contour integral representation, some multilinear and mixed multilateral generating functions; and, finally, we consider a singular integral equation with the function in the kernel and we provide its solution.
The celebrated Mittag-Leffler function [1, 2] is defined by
where ℂ denotes the set of complex numbers.
The Mittag-Leffler function arises naturally in the solution of fractional integral equations . A generalization of the Mittag-Leffler function has been investigated by Wiman . He studied the following function:
Other generalizations of the Mittag-Leffler functions were given in [5, 6]. Let us recall the one given by Srivastava and Tomovski :
where denotes the Pochhammer symbol defined, in terms of the Gamma function, by
where ℕ denotes the set of positive integers.
Multivariable analog of the Mittag-Leffler function has been introduced and investigated by Saxena et al. [, p.536, Eq. (1.14)] in the following form:
This function is, in fact, a special case of the generalized Lauricella series in several variables, introduced by Srivastava and Daoust  and Srivastava and Karlsson .
A mild generalization of the multivariable analog of the Mittag-Leffler function, which will play an important role in this paper, has been given by Saxena et al. [, p.547, Eq. (7.1)]:
Recently, the second author in  introduced a class of polynomials suggested by the multivariate Laguerre polynomials in the following form:
It is easy to see that the following relation between the class of polynomials given by (1.7) and the generalized multivariable Mittag-Leffler function (1.6) exists:
Note that by further specializing the several parameters involved, we can obtain many well-known classes of polynomials such as the Laguerre polynomials of r variables defined by Erdélyi  and the Konhauser polynomials .
Another interesting generalization of the polynomials is given by
Obviously, setting () leads to (1.8).
In this paper, we obtain a Schläfli’s type contour integral representation for the multivariable polynomials given in (1.9). Next, we give some multilinear and mixed multilateral generating functions. We also recall the fractional order integral of the generalized multivariable Mittag-Leffler function. Finally, we consider a singular integral equation with in the kernel and we give its solution. Throughout this paper, the variables are assumed to be real variables.
2 Schläfli’s type contour integral representation of
Let us define the following polynomials set:
The Schläfli’s type contour integral representation of in terms of is given in the next theorem.
Theorem 2.1 Let with () and let (). Then the following integral representation holds true:
Proof We have
With the help of Hankel’s formula 
we find from (2.3) and (2.4) the result asserted by Theorem 2.1. □
3 Multilinear and multilateral generating functions
We begin this section by proving a linear generating function for the polynomials by means of the mild generalization of the multivariate analog of Mittag-Leffler functions.
Theorem 3.1 We have
Proof Direct calculations yield
where we have interchanged the order of summations which is guaranteed because of the uniform convergence of the series under the conditions (). □
Now let , , , , , be complex j-tuples. By making use of the above theorem we have the following.
Theorem 3.2 Corresponding to an identically non-vanishing function of complex variables (), let
Suppose also that
provided that each member of (3.3) exists and ().
Proof Following similar lines to , the proof is completed. □
4 Fractional integrals and derivatives
In this section, we first recall the definitions of the Riemann-Liouville fractional integrals and derivatives. Next, we give the fractional integral and derivative of the generalized multivariable Mittag-Leffler function where are real variables for .
Definition 4.1 Let be a finite interval of the real axis. The Riemann-Liouville fractional integral of order with is defined by
It is well known [, p.71] that
Definition 4.2 Let be a finite interval of the real axis. The Riemann-Liouville fractional derivative of order with is defined by
where denotes the integral part of .
Using (4.2), we see easily that
Now, let us give two fractional calculus formulas obtained by Jaimini and Gupta [, p.145, Eqs. (1) and (2)] involving the generalized multivariable Mittag-Leffler function.
Theorem 4.3 Let such that ; ; ; (). Then the following fractional calculus formulas:
Setting , (), replacing , respectively, by , where () are positive integers in (4.5) and (4.6), and making use of (1.8) yield the following special cases given by Özarslan [, p.353, Theorem 6 and Theorem 8]:
Further special cases of (4.5) and (4.6) can be obtained by suitably specializing the coefficients involved. For instance, if we set (), then (4.5) and (4.6) reduce to two results obtained by Saxena et al. .
We end this section by giving a recurrence relation for the generalized multivariable Mittag-Leffler function .
Theorem 4.4 Let such that ; ; (). Then the following recurrence relation holds true:
Proof From (1.6), we have
5 Singular integral equation
In this section, we solve a singular integral equation with the generalized multivariable Mittag-Leffler function in the kernel. To do so, we first find the Laplace transform of the function and we compute an integral involving the product of two generalized multivariable Mittag-Leffler functions.
We denote the Laplace transform of a function f [, p.218] by
Lemma 5.1 Let such that ; ; ; ; (), we have
Proof Using (5.1), we get
where we used the well-known formula [, p.218, Eq. (3)]
Theorem 5.2 Let such that ; ; ; ; ; ; ; (), we have
Proof With the help of the convolution theorem for the Laplace transform (see )
From Lemma 5.1, we have
Finally, taking the inverse Laplace transform on both sides of (5.8), the result follows. □
Now, let us consider the following convolution equation involving the generalized multivariable Mittag-Leffler in the kernel:
Theorem 5.3 The singular integral equation (5.9) admits a locally integrable solution
provided that exists for and is locally integrable for .
Proof Applying the Laplace transform on both sides of (5.9), using the convolution theorem as well as Lemma 5.1, we find
which under the assumptions that can be rewritten as
Therefore, we have
Taking the inverse Laplace transform on both sides of (5.13) and with the help of the following property [, p.217, Eq. (3.8)]:
which holds for suitable f, we thus obtain
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The authors declare that they have no competing interests.
All authors completed the paper together. All authors read and approved the final manuscript.
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Gaboury, S., Özarslan, M.A. Singular integral equation involving a multivariable analog of Mittag-Leffler function. Adv Differ Equ 2014, 252 (2014). https://doi.org/10.1186/1687-1847-2014-252
- fractional integrals and derivatives
- Mittag-Leffler function
- contour integral representation
- generating functions
- singular integral equation
- Laplace transform