In this paper, the Bromwich integral for the inverse Mellin transform is used for finding an integral representation for a fractional exponential operator. This operator can be considered as an approach for solving partial fractional differential equations. Also, application of this operator for obtaining a formal solution of the time-fractional telegraph equation is discussed.
MSC: 26A33, 35A22, 44A10.
Keywords:Laplace transform; Mellin transform; partial fractional differential equation; Wright function
1 Introduction and problem
We consider the exponential operator
where , are specified by the system of first-order differential equations 
By the above exponential operator, Dattoli et al. found solutions of some boundary value problems arising in mathematical physics in terms of integral transforms type; see [2,3] and references therein. Also, they used this operational technique to describe properties of some special polynomials and functions [4-6]; also see .
When we encounter an exponential operator of higher order , where α is integer or non-integer and , it is of interest to have an integral representation to reduce the order and apply the relation (1.1). For example, for exponential operators of orders two and three, we can write the Gauss-Weierstrass and the Airy integrals [2,7]
where is the Airy function of the first kind given by
where the Wright function is presented by the following relation :
In this paper, in a general case we obtain an integral representation for , , with order one for s, and then we show how this operator can be applied to find the formal solutions of partial fractional differential equations (PFDEs).
This problem for integral representation is referred to as the inverse of the Mellin transform of , , and in Section 2, we state main theorems and corollaries related to it. In Section 3, as an application of this technique, we find formal solutions of the space-fractional Moshinskii’s equation and the time-fractional telegraph equation. Finally, in Section 4 the main conclusions are drawn.
2 Main theorems and corollaries
In this section, we establish some theorems on the fractional exponential operator which can be useful for solving PFDEs. First, we derive an integral representation for the operator , which can be considered as a generalized representation of the relations (1.3) and (1.4).
Theorem 2.1The following identity holds true for :
where the function is presented by
Proof By the definition of the inverse of the Mellin transform for a function , we have
The above relation implies that the Mellin transform of the last integral is equal to the function , that is,
By setting , we get the relation (2.1). □
Theorem 2.2 (The Schouten-Van der Pol theorem for the Laplace transform )
Letcbe a suitable real constant such that and are analytic functions in the half-plane and is the Laplace transform of . Then the inverse of the Laplace transform is given by
Proof Using the definition of the Laplace transform for
replacing in the inverse of the Laplace transform
and changing the order of integration, we get the relation (2.4). □
Corollary 2.3It is obvious that by setting , , in the relations (2.4) and using the relation (1.6) for the inverse of the Laplace transform , the inverse of the Laplace transform can be presented by
Corollary 2.4By setting and combining the relations (2.1) and (2.5), we get a new integral representation for the fractional exponential equation
where the function is given by
In view of the theorems of a fractional exponential operator expressed in this section, we may apply this operator to PFDEs in the next section.
3 Application to partial fractional differential equations
Example 3.1 In connection with initial-value diffusions, we consider the space-fractional Moshinskii’s equation of order γ in the Riemann-Liouville sense 
with the Cauchy-type initial condition as .
In order to obtain the solution of (3.1), by solving the first-order partial differential with respect to t and applying the initial condition, the formal solution in the form of fractional exponential operator gives rise to
Now, by setting , and applying Corollary 2.4 for the integral representation of , we can write the solution in terms of the integral transform as
where the function is given by the relation (2.7). The above relation can be simplified in the following form:
where we used the relations (1.1) and (1.2) by choosing the functions and .
with initial and asymptotic conditions , .
Similar to the previous problem by solving the equation with respect to x and applying the initial and asymptotic conditions, the formal solution takes the form:
Now, by setting and writing an integral representation for in terms of the Bessel function of order one, we get 
We can rewrite the relation (3.5) in the following form:
where we used the relation (2.6) for the linearization of a fractional exponential operator , and then we applied the relations (1.1) and (1.2) by substituting and .
This paper provides some new results in the theory of fractional derivative. These results show the flexible operational technique can be used in a fairly wide context beside the integral transforms for obtaining the formal solutions of PFDEs.
Also, this technique can be considered as a promising approach for many applications in applied sciences.
The author declares that he has no competing interests.
The author was partially supported by the Center of Excellence for Mathematics, University of Shahrekord.
Dattoli, G, Ottaviani, PL, Torte, A, Vazquez, L: Evolution operators equations: integration with algebraic and finite difference methods. Applications to physical problems in classical and quantum mechanics and quantum field theory. Riv. Nuovo Cimento. 20, 1–133 (1997)
Dattoli, G, Srivastava, HM, Zhukovsky, Z: Operational methods and differential equations with applications to initial-value problems. Appl. Math. Comput.. 184, 979–1001 (2007). Publisher Full Text
Dattoli, G, Ricci, PE, Khomasuridze, I: Operational methods, special polynomial and functions and solution of partial differential equations. Integral Transforms Spec. Funct.. 15(4), 309–321 (2004). Publisher Full Text
Babusci, D, Dattoli, G, Sacchetti, D: The Airy transform and the associated polynomials. Cent. Eur. J. Phys.. 9(6), 1381 (2011). Publisher Full Text
Dattoli, G: Operational methods, fractional operators and special polynomials. Appl. Math. Comput.. 141, 151–159 (2003). Publisher Full Text
Dattoli, G, Ricci, PE, Sacchetti, D: Generalized shift operators and pseudo-polynomials of fractional order. Appl. Math. Comput.. 141, 215–224 (2003). Publisher Full Text
Aghili, A, Ansari, A: Solving partial fractional differential equations using the -transform. Asian-Eur. J math.. 3(2), 209–220 (2010). Publisher Full Text
Aghili, A, Ansari, A: Solution to system of partial fractional differential equation using the -transform. Anal. Appl.. 9(1), 1–9 (2011). Publisher Full Text
Ansari, A, Refahi Sheikhani, A, Saberi Najafi, H: Solution to system of partial fractional differential equation using the fractional exponential operators. Math. Methods Appl. Sci.. 35, 119–123 (2012). Publisher Full Text
Chen, J, Liu, F, Anh, V: Analytical solution for the time-fractional telegraph equation by the method of separating variables. J. Math. Anal. Appl.. 338, 1364–1377 (2008). Publisher Full Text
Orsingher, E, Beghin, L: Time-fractional telegraph equations and telegraph processes with Brownian time. Probab. Theory Relat. Fields. 128, 141–160 (2004). Publisher Full Text