It is well known that seeking exact solutions for nonlinear evolution equations (NLEES) plays an important role in mathematical physics. In the past five decades or so, many effective methods have been presented, which contain the inverse scattering method [1] , Hirota bilinear method [2] , the tanh-function method and its various extension [3] [4] , the sine-cosine function method [5] [6] , homogeneous balance method [7] [8] , Jacobi elliptic function method [9] [10] , the first-integral method [11] [12] , the sine-Gordon equation method [13] and the Exp-function method [14] -[16] .

Recently, Wang et al. [17] proposed a new method called the -expansion method to look for travelling

wave solutions of nonlinear evolution equations (NLEEs). The -expansion method is based on the assump-

tions that the travelling wave solutions can be expressed by a polynomial in as follows:

where satisfies a second order linear ordinary differential equation (LODE):

where, , , and is a constant. The degree of the polynomial can be de-

termined by considering the homogeneous balance between the highest order derivative and nonlinear terms ap- pearing in the given NLEE. The coefficients of the polynomial can be obtained by solving a set of algebraic eq-

uations resulted from the process of using the method. By using the -expansion method, many nonlinear

equations [17] -[27] have been successfully solved.

In the present paper, we will extend the -expansion method to the following generalized Benjamin equa-

tion:

where and are constants. Equation (3) is used in the analysis of long wave in shallow water; see [28] . To the best of our knowledge, there are a few articles about this equation. Recently, by applying the extended tanh method, Taghizadeh et al. [29] obtained some exact solutions. In the subsequent section, we will illustrate

the -expansion method in detail with the generalized Benjamin equation.

First, in this section, we start out our study for Equation (3). Firstly, making the following wave variable

where, and are constants to be determined later, Equation (3) becomes the ODE

where the prime denotes the derivation with respect to. Integrating Equation (5), twice and setting the con- stant of integrating to zero, we obtain

Then, making the following transformation

We can obtain an equation for as

Now, we make an ansatz (1) for the solution of Equation (8). Balancing the terms and in Equation (8) yields the leading order. Therefore, we can write the solution of Equation (8) in the form

Substituting (2) and (9) into (8), collecting the coefficients of and set it to zero we ob-

tain the following system of algebraic equations for:, , , , , and:

Solving the above system by Matlab gives

where, , and are arbitrary constants.

Substituting (10) (11) into (9) yields:

where.

Substituting the general solutions of Equation (2) into the formulae (12) we have three types of travelling wave solutions of the generalized Benjamin equation as follows:

When,

where, , and are arbitrary constants. It is

easy to see that the hyperbolic solution (13) can be rewritten at, as follows

while at, one can obtain

where.

When,

where, , and are arbitrary constants. Simila-

rity, it is easy to see that the trigonometric solution (16) can be rewritten at and, as follows

and

where.

When,

where, and are arbitrary constants.

Authors [29] have looked for exact solutions of Equation (3) using the wave variable, Equa- tion (3) should become the ODE

not the ODE (7) in [29] in the following form

Therefore, the exact solutions given in [29] are wrong. To the best of our knowledge, solutions (13), (16) and (19) have not been reported in literature.

The -expansion method has been successfully applied here to seek exact solutions of the generalized Ben-

jamin equation. As a result, a series of new exact solutions are obtained. The solution procedure is very simple and the travelling wave solutions are expressed by the hyperbolic functions, the trigonometric functions and the rational functions. To the best of our knowledge, these solutions have not been reported in literature. It is shown

that the -expansion method provides a very effective and powerful mathematical tool for solving nonlinear

equations in mathematical physics.

This work was supported by the research project of Yuncheng University (No.YQ-2011013, YQ-2011068, XK- 2012004).