#### Abstract

We present a new technique to obtain the solution of time-fractional coupled Schrödinger system. The fractional derivatives are considered in Caputo sense. The proposed scheme is based on Laplace transform and new homotopy perturbation method. To illustrate the power and reliability of the method some examples are provided. The results obtained by the proposed method show that the approach is very efficient and simple and can be applied to other partial differential equations.

#### 1. Introduction

The intuitive idea of fractional order calculus is as old as integer order calculus. It can be observed from a letter that was written by Leibniz to * ĹHôpital*. The fractional order calculus is a generalization of the integer order calculus to a real or complex number. Fractional differential equations are used in many branches of sciences, mathematics, physics, chemistry, and engineering. Applications of fractional calculus and fractional-order differential equations include dielectric relaxation phenomena in polymeric materials [1], transport of passive tracers carried by fluid flow in a porous medium in groundwater hydrology [2], transport dynamics in systems governed by anomalous diffusion [3, 4], and long-time memory in financial time series [5] and so on [6, 7]. In particular, recently, much attention has been paid to the distributed-order differential equations and their applications in engineering fields that both integer-order systems and fractional-order systems are special cases of distributed-order systems. The reader may refer to [8–10].

Several schemes have been developed for the numerical solution of differential equations. The homotopy perturbation method was proposed by He [11] in 1999. This method has been used by many mathematicians and engineers to solve various functional equations. Homotopy method was further developed and improved by He and applied to nonlinear oscillators with discontinuities [12], nonlinear wave equations [13], and boundary value problems [14]. It can be said that He’s homotopy perturbation method is a universal one and is able to solve various kinds of nonlinear functional equations. For example, it was applied to nonlinear Schrödinger equations [15], to nonlinear equations arising in heat transfer [16], and to other equations [17–20]. In this method, the solution is considered to be an infinite series which usually converges rapidly to exact solutions. In this paper we introduce a new form of homotopy perturbation and Laplace transform methods by extending the idea of [21].

We extend the homotopy perturbation and Laplace transform method to solve the time-fractional coupled Schrödinger system. The nonlinear time-fractional coupled Schrödinger partial differential system is as [22] where are unknown functions, are real constants, and is a parameter describing the order of the fractional Caputo derivative. and are arbitrary (smooth) nonlinear real functions. Nonlinear Schrödinger system is one of the canonical nonlinear equations in physics, arising in various fields such as nonlinear optics, plasma physics, and surface waves [23].

This paper is organized as follows. In Section 2, we recall some basic definitions and results dealing with the fractional calculus and Laplace transform which are later used in this paper. In Section 3 the homotopy perturbation method is described. The basic idea behind the new method is illustrated in Section 4. Finally, in Section 5, the application of homotopy perturbation and Laplace transform method for solving time-fractional coupled Schrödinger systems are presented.

#### 2. Preliminaries and Notations

Some basic definitions and properties of the fractional calculus theory are used in this paper.

*Definition 1. *A real function , , is said to be in the space , , if there exists a real number such that , where and it is said to be in the space if and only if , . Clearly if .

*Definition 2. *The left-sided Riemann-Liouville fractional integral operator of order , of a function , , is defined as follows:
where is the well-known Gamma function.

Some of the most important properties of operator , for , , and are as follows:

*Definition 3. *Amongst a variety of definitions for fractional order derivatives, Caputo fractional derivative has been used [24, 25] as it is suitable for describing various phenomena, since the initial values of the function and its integer order derivatives have to be specified, so Caputo fractional derivative of function is defined as
where , , , and .

In this paper, we have considered time-fractional coupled Schrödinger system, where the unknown function is assumed to be a causal function of fractional derivatives which are taken in Caputo sense as follows.

*Definition 4. *The Caputo time-fractional derivative operator of order is defined as

*Definition 5. *The Laplace transform of a function , is defined as
where can be either real or complex. The Laplace transform of the Caputo derivative is defined as

Lemma 6. *If , , and , then we have
**
where is the inverse Laplace transform.*

The Mittag-Leffler function plays a very important role in the fractional differential equations and in fact it was introduced by Mittag-Leffler in 1903 [26]. The Mittag-Leffler function with is defined by the following series representation: where . For , (9) becomes The key result that indicates why Mittag-Leffler functions are so important in fractional calculus is the following theorem. It essentially states that the eigenfunctions of Caputo differential operators may be expressed in terms of Mittag-Leffler functions.

Theorem 7 (see [27]). *If and , then we have
*

#### 3. The Homotopy Perturbation Method

For the convenience of the reader, we will first present a brief account of homotopy perturbation method. Let us consider the following differential equation: with boundary conditions where is a general differential operator, is a boundary operator, is a known analytic function, and is the boundary of the domain .

The operator can be generally divided into two parts and , where is linear, while is nonlinear. Therefore, (12) can be written as follows: By using homotopy technique, one can construct a homotopy which satisfies which is equivalent to where is an embedding parameter and is an initial guess approximation of (12) which satisfies the boundary conditions. Clearly, we have Thus, the changing process of from to is just that of from to . In topology this is called deformation and and are called homotopic. If, the embedding parameter , is considered as a small parameter, applying the classical perturbation technique, we can naturally assume that the solution of (15) and (16) can be given as a power series in ; that is, According to homotopy perturbation method, the approximation solution of (12) can be expressed as a series of the power of ; that is, The convergence of series (19) has been proved by He in his paper [11]. It is worth noting that the major advantage of homotopy perturbation method is that the perturbation equation can be freely constructed in many ways by homotopy in topology and the initial approximation can also be freely selected. Moreover, the construction of the homotopy for the perturbed problem plays a very important role for obtaining desired accuracy [28].

#### 4. Basic Ideas of the Homotopy Perturbation and Laplace Transform Method

To illustrate the basic ideas of this method, we consider the general form of a system of nonlinear fractional partial differential equations: with initial conditions where are operators and are known analytical functions. The operators can be divided into two parts, and , where are the linear operators and are nonlinear operators. Therefore, (20) can be rewritten as By the new homotopy perturbation method [29], we construct the following homotopies: or equivalently where is an embedding parameter and are initial approximations for the solution of (20). Clearly, we have from (23) and (24) By applying Laplace transform on both sides of (24), we have Using (7), we derive or By applying inverse Laplace transform on both sides of (28), we have According to the homotopy perturbation method, we can first use the embedding parameter as a small parameter and assume that the solution of (29) can be written as a power series in as follows: where , , are functions which should be determined. Suppose that the initial approximation of the solutions of (20) is in the following form: where , for , , are functions which must be computed. Substituting (30) and (31) into (29) and equating terms with identical powers of we obtain the following set of equations:

Now if we solve these equations in such a way that , then (32) yield Therefore the exact solution is obtained by

#### 5. Example

To illustrate the power and reliability of the method for the time-fractional coupled Schrödinger system some examples are provided. The results reveal that the method is very effective and simple.

*Example 8. *Consider the following linear time-fractional coupled Schrödinger system:
subject to the following initial conditions:
where and are of the form
with the exact solutions
where .

To solve (35) by the homotopy perturbation and Laplace transform method, we construct the following homotopy:
Applying the Laplace transform on both sides of (39), we have
or
The inverse Laplace transform of (41) and the initial conditions lead us to
Suppose that the solution is expanded as (30); substituting (30) into (42), collecting the same powers of , and equating each coefficient of to zero yield

Assume , . Solving the above equations, for , , leads to the result
By the vanishing of , the coefficients , are determined to be
Therefore, the solutions of (35) are
which are the exact solutions. Now, if we put in (46), we obtain , which is the exact solution of the given coupled Schrödinger system (35).

*Example 9. *Consider the following nonlinear time-fractional coupled Schrödinger system:
subject to the following initial conditions:
where and have the following form:
with the exact solutions
where .

To solve (47) by the homotopy perturbation and Laplace transform method, we construct the following homotopy:
Applying Laplace transform on both sides of (51), we have
or
The inverse Laplace transform of (53) and the initial conditions lead us to
Suppose that the solution is expanded as (30); substituting (30) into (54), collecting the same powers of , and equating each coefficient of to zero yield

Assume , . Solving the above equations, for , , leads to the result
By the vanishing of , the coefficients , are determined to be
Therefore, the exact solutions of the system of (47) can be expressed as
Now, if we put in (58), we obtain , which is the exact solution of the given coupled Schrödinger system (47).

*Example 10. *Consider the following nonlinear time-fractional coupled Schrödinger system:
subject to the following initial conditions:
where and have the following form:
with the exact solutions
where .

To solve (59) by the LTNHPM, we construct the following homotopy:
Applying Laplace transform on both sides of (63), we have
or
The inverse Laplace transform (65) and the initial conditions , , lead us to
Suppose that the solution is expanded as (30); substituting (30) into (66), collecting the same powers of , and equating each coefficient of to zero yield

Assume , . Solving the above equations, for , , we obtain the result
By the vanishing of , the coefficients , are determined to be
Therefore, the solutions of (59) are