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Jacobi elliptic function solutions for a two-mode KdV equation

Department of Mathematics and Statistics, Jordan University of Science and Technology, P.O.Box(3030), Irbid 22110, Jordan

⁎Corresponding author. marwan04@just.edu.jo (Marwan Alquran)

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

In this paper we introduce a new type of KdV equations called Two-mode KdV (TMKdV). This equation represents the propagation of two-wave modes in the same direction simultaneously. We suggest a finite series of degree n in terms of a Jacobi elliptic functions as a possible solution for the TMKdV. We succeeded in obtaining new solutions of type solitons and periodics.

Keywords

Jacobi elliptic function methods
Two-mode KdV equation
35C08
74J35
PubMed
1

1 Introduction

Most of nonlinear equations are defined by first-order partial differential equation (PDE) in time. They describe unidirectional waves where these equations model one right-moving for x>o . Such equations is the well-known KdV. Other equations model two left-right-moving as in Boussinesq which is defined by second-order PDE in time. Therefore, Korsunsky (1994) developed this phenomena and he was able to identify and derive two-mode KdV (TMKdV) as a nonlinear PDE of second order in time. This two-mode equation represents the propagation of two-wave modes in the same direction simultaneously. The TMKdV reads as

(1.1)
0=utt+(c1+c2)uxt+c1c2uxx+(α1+α2)t+(α1c2+α2c1)xuux+(β1+β2)t+(β1c2+β2c1)xuxxx, where u(x,t) is a field function, -<x,t<,αi-1 and βi<1 . The ci are the phase velocities, the αi are the parameters of nonlinearity, and the βi are the dispersion parameters ore (modes). The field function u(x,t) represents the height of the water’s free surface above a flat bottom. The TMKdV equation describes the propagation of two different wave modes in the same direction simultaneously, with the same dispersion relation but different phase velocities, nonlinearity, and dispersion parameters (Korsunsky, 1994; Xiao et al., 2016; Lee and Liu, 2011; Lee et al., 2010; Lee and Lee, 2013; Zhu et al., 1997; Hong and Jung, 1999).

The TMKdV Eq. (1.1) was studied recently in the literature. Analytic methods for this equation have been used, such as the reductive perturbation (Lee and Liu, 2011), Hamiltonian system (Lee and Lee, 2013), Bell polynomials (Hong and Jung, 1999), and others. In Xiao et al. (2016), it was found that both the two solitons in the two modes separate without any change of their initial shapes and velocities except for the phase shifts after each collision. In Lee et al. (2010), three basic conserved quantities, namely, the mass, momentum, and Hamiltonian for the TMKdV equation were constructed by using the prolongation method.

Many models in mathematics and physics are described by nonlinear partial differential equations (NPDEs). The theory of solitary wave solution has contributed to understanding many experiments and complex phenomena in mathematical physics. Thus, the literature has been enriched by well imposed methods that produce exact solutions of such nonlinear equations. Such methods are: The sine-cosine method (Wazwaz, 2004; Alquran and Qawasmeh, 2013; Alquran, 2012), Rational sine-cosine (Qawasmeh and Alquran, 2014), tanh method (Alquran and Al-Khaled, 2011), extended tanh method (Shukri and Al-khaled, 2010), sech-tanh method (Alquran et al., 2012), Exp-function method (Raslan, 2009), the (G/G) -expansion method (Alquran and Qawasmeh, 2014; Qawasmeh and Alquran, 2014) and so on. The aforementioned methods can be classified in two categories; special structure in terms of trigonometric or hyperbolic functions or a finite series of degree n in terms of a trigonometric-hyperbolic function. The order of this finite series is to be determined by a balance procedure. Finally, some new and important developments for searching for analytical solitary waves solutions for PDEs have been added recently to the literature (Seadawy, 2014, 2015, 2016, 2017; Helal and Seadawy, 2009; Seadawy and El-Rashidy, 2013).

The goal of this work is to explore more new exact solutions of the two-mode KdV equation given in (1.1) by using a finite series of degree n in terms of a Jacobi elliptic functions (Liu et al., 2001; Fu et al., 2001) which belongs to the well-known homogeneous balance principle and truncated function expansion method (Fan and Zhang, 1998, 2002; Dai and Zhang, 2006; Liu et al., 2001).

2

2 Jacobi elliptic sine and cosine functions

In this section we introduce in details the derivation of Jacobi elliptic functions. First, we consider the second order partial deferential equation (PDE)

(2.1)
2ϕxt=αsin(ϕ).

Applying the linear transformation ξ=k(x-λt) to the above PDE leads to the following ordinary differential equation (ODE)

(2.2)
ϕ=-αk2λsin(ϕ), which is equivalent to
(2.3)
12ϕ2=-αk2λsin212(ϕ)+c.

By forcing c=1,-αk2λ=-m2 and w=12(ϕ) , we write (2.3) as

(2.4)
(w)2=1-m2sin2w, or
(2.5)
w=1-m2sin2w.

Separating the variables in (2.5) leads to the following integral

(2.6)
11-m2sin2wdw=dξ, which is known as the legendre elliptic integral of first kind and m(0,1) is a parameter which is known as the module.

Now, we define

(2.7)
u=u(t)=0ϕ11-m2sin2ydy=0tsiny1(1-x2)(1-m2x2)dx, and we propose that u=f(t)t=f-1(u)=sn(u) . Where sn(u) is called the Jacobi elliptic sine function. To define the Jacobi elliptic cosine function, we let
(2.8)
u(t)=0ϕ11-m2(1-cos2y)dy,
and we set 1-t2=cosy to get
(2.9)
u(t)=01-t2cosy1(1-x2)(1-m2x2)dx.

Therefore, u=f(1-t2)1-t2=f-1(u)=cn(u) and cn(u) is called the Jacobi elliptic cosine function. Based on the above two definitions, we may write the following argument

(2.10)
t=sn(u)1-t2=cn(u)1-m2t2=dn(u).

Thus, we reach to the identities, cn2(u)=1-sn2(u) and dn2(u)=1-m2sn2(u) . To know more about these functions, we study their derivatives. First we start with sn(u) ,

(2.11)
u=dt(1-t2)(1-m2t2)du=dt(1-t2)(1-m2t2).

Thus,

(2.12)
dtdu=(1-t2)(1-m2t2).

Now we are ready ti introduce the following formulas

(2.13)
ddu(sn(u))=ddu(t)=(1-t2)(1-m2t2)=cn(u)dn(u)ddu(cn(u))=ddu(1-sn2(u))=-2sn(u)21-sn2(u)cn(u)dn(u)=-sn(u)dn(u)ddu(dn(u))=ddu(1-m2sn2(u))=-2m2sn(u)21-m2sn2(u)cn(u)dn(u)=-m2sn(u)cn(u).

Also, we can observe by the second equation of (2.13) that,

(2.14)
ddu(cn(u))=-sn(u)dn(u)=-1-cn2(u)1-m2sn2(u)=-1-cn2(u)1-m2+m2cn2(u).

Therefore, cn(u)=-0tdx(1-x2)(1-m2+m2x2) . On the other hand, this can be obtained by substituting t=cosy in the elliptic integral

(2.15)
0ϕdy1-m2sin2y=0ϕdy1-m2(1-cos2y).

Equivalently,

(2.16)
-0tcosydx(1-x2)(1-m2+m2x2)=f(t).

Thus, u=f(t)t=f-1(u)=cn(u) .

Also, sn(u)=t=f-1(u) , where

(2.17)
u=u(t)=0tdx(1-x2)(1-m2x2).

Now, if m1 , then the integral become

(2.18)
0tdx(1-x2)=12(ln(|t+1|)-ln(|1-t|))=tanh-1(t), and leads to:
(2.19)
sn(u)=tanh(u)cn(u)=1-sn2(u)=1-tanh2(u)=sech(u)dn(u)=1-sn2(u)=sech(u).

Figs. 1 and 2 represent the functions sn(x,m) and cn(x,m) for special values of the index m.

Plots of the Jacobi sn ( x , m ) function sn ( x , 0 ) = sin ( x ) , sn ( x , 1 2 ) , sn ( x , 1 ) = tanh ( x ) respectively and - 10 < x < 10 .
Fig. 1
Plots of the Jacobi sn(x,m) function sn(x,0)=sin(x),sn(x,12),sn(x,1)=tanh(x) respectively and -10<x<10 .
Plots of the Jacobi cn ( x , m ) function cn ( x , 0 ) = cos ( x ) , cn ( x , 1 2 ) , cn ( x , 1 ) = sech ( x ) respectively and - 10 < x < 10 .
Fig. 2
Plots of the Jacobi cn(x,m) function cn(x,0)=cos(x),cn(x,12),cn(x,1)=sech(x) respectively and -10<x<10 .

Now, we will highlight briefly the main steps of the Jacobi elliptic sine-cosine expansion method. We first unite the independent variables x and t into one wave variable ζ=x-ct to convert the partial differential equation (PDE)

(2.20)
P(u,ut,ux,uxt,) into an ordinary differential equation (ODE)
(2.21)
Q(u,-cu,u,-cu,).

Eq. (2.21) is then integrated as long as all terms contain derivatives. The Jacobi elliptic sine-cosine technique is based on the assumption that the traveling wave solutions can be expressed in terms of the sn(ξ,m) or cn(ξ,m) . We therefore introduce a new independent variable

(2.22)
Y=sn(μζ,m), or
(2.23)
Y=cn(μζ,m).

Then, the solution can be proposed a finite power series in Y in the form:

(2.24)
u(ζ)=S(Y)=i=0MaiYi.

The parameter M is a positive integer, in most cases, that will be determined by using a balance procedure, where by comparing the behavior of Yi in the highest derivative against its counterpart within the nonlinear terms. With M determined, we collect all coefficients of powers of Y in the resulting equation where theses coefficients have to vanish, hence the coefficients ai can be determined.

3

3 Solutions of the two-mode KdV equation

In this section, we apply the Jacobi elliptic sine-cosine expansion method to extract possible solutions of (1.1). By using the wave variable ξ=k(x-λt) and chain rule, the TMKdV equation is reduced to the following ordinary equation:

(3.1)
0=(c2α1+c1α2-(α1+α2)λ)u2+(c1-λ)(c2-λ)u+(c2α1+c1α2-(α1+α2)λ)uu+k2(c2β1+c1β2-(β1+β2)λ)u, where u=u(ξ) . Balancing the linear term u with the nonlinear term u2 produces the algebraic equation n+4=2n+2 whose solution is n=2 . Thus, by using the Jacobi sine function, the solution of (3.1) has the form
(3.2)
u(ξ)=a0+a1sn(ξ,m)+a2sn2(ξ,m).

To determine the coefficients a0,a1,a2 and the wave’s parameters λ and k, we substitute (3.2) into (3.1) and collect the coefficients for the same power of Jacobi sine function and set them to zero to obtain the following system:

(3.3)
0=10a2m2(a2(c2α1+c1α2-(α1+α2)λ)+12k2m2(c2β1+c1β2-(β1+β2)λ))0=2a2(-4a2(1+m2)(c2α1-(α1+α2)λ)+c1(3c2m2-4a2(1+m2)α2-3m2(-a0α2+20k2(1+m2)β2+λ))+3m2(-c2(20k2(1+m2)β1+λ)+λ(20k2(1+m2)(β1+β2)+λ)+a0(c2α1-(α1+α2)λ)))0=-2a2(-3a2c2α1-8c2k2β1-52c2k2m2β1-8c2k2m4β1-2c2λ-2c2m2λ+3a2α1λ+3a2α2λ+8k2β1λ+52k2m2β1λ+8k2m4β1λ+8k2β2λ+52k2m2β2λ+8k2m4β2λ+2λ2+2m2λ2+c1(2c2(1+m2)-3a2α2+2a0(1+m2)α2-8k2β2-52k2m2β2-8k2m4β2-2λ-2m2λ)+2a0(1+m2)(c2α1-(α1+α2)λ))0=2a2(-4c2k2β1-4c2k2m2β1+c1(c2+a0α2-4k2β2-4k2m2β2-λ)-c2λ+4k2β1λ+4k2m2β1λ+4k2β2λ+4k2m2β2λ+λ2+a0(c2α1-(α1+α2)λ))

Some of the equation in the above system have been deleted where a1=0 is the solution of each. Now, solving this algebraic system yields

(3.4)
a0=4k2(1+m2)(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λa2=-12k2m2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λ.

Therefore, the Jacobi sine solution of the two-mode KdV equation is

(3.5)
u(x,t)=4k2(1+m2)(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λ-12k2m2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λsn2(k(x-λt),m).

If m=1 in (3.5), then we obtain the hyperbolic tangent solution

(3.6)
u(x,t)=8k2(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λ-12k2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λtanh2(k(x-λt)), where k and λ are free constants.

Now we proceed as above but replacing the Jacobi sine by Jacobi cosine function in (3.2), i.e.,

(3.7)
u(ξ)=b0+b1cn(ξ,m)+b2cn2(ξ,m).

Following the procedures considered as in the Jacobi Sine case, one can reach to the following results

(3.8)
b0=-4k2(2m2-1)(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λb1=0b2=12k2m2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λ.

Therefore, the Jacobi cosine solution of the two-mode KdV equation is

(3.9)
u(x,t)=-4k2(2m2-1)(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λ+12k2m2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λcn(k(x-λt)).

If m=1 in (3.9), then we obtain the hyperbolic secant solution

(3.10)
u(x,t)=-4k2(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λ+12k2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λsech2(k(x-λt)), where k and λ are free constants.

Finally, if we use the Jacobi DN Solution to the two-mode KdV equation, we get the same obtained solution as given in (3.9) but with value of λ of 1.

4

4 Discussion and concluding remarks

In this work, we succeeded in obtaining Jacobi elliptic sine-cosine solutions for the proposed two-mode KdV Eq. (1.1). We may reduce the calculations performed in the previous section and obtain the Jacobi cosine solution given in (3.9) by using the identity sn2(ξ,m)+cn2(ξ,m)=1 . Indeed, if we replace sn2(ξ,m) in (3.5) by 1-cn2(ξ,m) , then b0=a0+a2 and b2=-a2 .

The types of the obtained solutions given in (3.6) and (3.10) are solitons. Solitons are solutions in the form sech and sech2(tanh2) , the graph of a Soliton is a wave where in the limit x± , the answer is 0.

By using the facts tanh(ix)=itan(x) and sech(ix)=sec(x) , if we let k=iμ in Eqs. (3.6) and (3.10), then two more periodic solutions will be obtained

(4.1)
u(x,t)=-8μ2(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λ-12μ2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λtan2(μ(x-λt)), and
(4.2)
u(x,t)=4μ2(c2β1+c1β2-(β1+β2)λ)+λ(c1+c2)-c1c2-λ2c2α1+c1α2-(α1+α2)λ-12μ2(c2β1+c1β2-(β1+β2)λ)c2α1+c1α2-(α1+α2)λsec2(μ(x-λt)),
where μ and λ are free constants. Finally, the last two periodic solutions have singularities at the characteristic lines μ(x-λt)=π2±nπ (Fig. 3).
The obtained solutions of the TMKdV given in (3.6) and (4.1) respectively when c 1 = 0.5 , c 2 = 1 3 . α 1 = 1 , α 2 = 0.8 , β 1 = 0.25 , β 2 = 0.5 , k = μ = 1 and λ = 1 .
Fig. 3
The obtained solutions of the TMKdV given in (3.6) and (4.1) respectively when c1=0.5,c2=13 . α1=1,α2=0.8,β1=0.25,β2=0.5,k=μ=1 and λ=1 .

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