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# The general meromorphic solutions of the Petviashvili equation

- Zifeng Huang
^{1, 2}, - Wenjun Yuan
^{1}Email author and - Jinchun Lai
^{1}

**2014**:24

https://doi.org/10.1186/1687-1847-2014-24

© Huang et al.; licensee Springer. 2014

**Received: **21 September 2013

**Accepted: **19 December 2013

**Published: **17 January 2014

## Abstract

In this paper, we employ the complex method to first obtain all meromorphic exact solutions of complex Petviashvili equation, and then find all exact solutions of Petviashvili equation. The idea introduced in this paper can be applied to other non-linear evolution equations. Our results show that the complex method is simpler than other methods. Finally, we give some computer simulations to illustrate our main results.

**MSC:**30D35, 34A05.

## Keywords

- Petviashvili equation
- exact solution
- meromorphic function
- elliptic function

## 1 Introduction and main results

In 2006 and 2008, Zhang *et al.* [1, 2] obtained abundant exact solutions of the Petviashvili equation by using the modified mapping method and the availability of symbolic computation. These solutions include the Jacobi elliptic function solutions, triangular function solutions, and soliton solutions. In this paper, we employ the complex method to obtain first all traveling meromorphic exact solutions of complex Petviashvili equation, and then find all exact solutions of the Petviashvili equation.

In order to state our main result, we need some concepts and some notation. A meromorphic function $w(z)$ means that $w(z)$ is holomorphic in the complex plane ℂ except for poles. *α*, *b*, *c*, ${c}_{i}$, and ${c}_{ij}$ are constants, which may be different from each other in different places. We say that a meromorphic function *f* belongs to the class *W* if *f* is an elliptic function, or a rational function of ${e}^{\alpha z}$ ($\alpha \in \mathbb{C}$), or a rational function of *z*.

are two-dimensional Laplace and Jacobian operators, respectively, ${C}_{R}$ is the linear zero-dimensional phase velocity of Rossby wave, ${L}_{R}$ is the characteristic length about *x* and *y*, *H* is the average thickness of the fluid, and *g* is the acceleration of gravity; $\beta =\frac{\partial f}{\partial y}$, and $\frac{1}{f}$ is the characteristic value about *t*.

where *ω*, *k*, *l*, ${C}_{R}$ are constants.

Our main result is Theorem 1.

**Theorem 1**

*Equation*(1)

*is integrable if and only if*$2dk{C}_{R}=-12{\omega}^{2}{({k}^{2}+{l}^{2})}^{2}{g}_{2}+{(\omega +k{C}_{k})}^{2}$, ${F}^{2}=4{E}^{3}-{g}_{2}E-{g}_{3}$, ${g}_{3}$,

*and*

*E*

*are arbitrary*.

*Furthermore*,

*the general solutions of the Eq*. (1)

*are of the following form*.

- (I)
*The elliptic general solutions are*$\begin{array}{rcl}{w}_{d}(z)& =& \frac{12\omega ({k}^{2}+{l}^{2})}{k{C}_{R}}\{-\mathrm{\wp}(z)+\frac{1}{4}{\left[\frac{{\mathrm{\wp}}^{\prime}(z)+F}{\mathrm{\wp}(z)-E}\right]}^{2}\}\\ -\frac{12\omega ({k}^{2}+{l}^{2})E}{k{C}_{R}}-1-\frac{\omega}{k{C}_{R}},\end{array}$(2)

*if*$2dk{C}_{R}=-12{\omega}^{2}{({k}^{2}+{l}^{2})}^{2}{g}_{2}+{(\omega +k{C}_{k})}^{2}$, ${F}^{2}=4{E}^{3}-{g}_{2}E-{g}_{3}$, ${g}_{3}$

*and*

*E*

*are arbitrary*.

- (II)
*The simply periodic solutions are*${w}_{s}(z)=\frac{3\omega ({k}^{2}+{l}^{2})}{k{C}_{R}}\alpha {coth}^{2}\frac{\alpha}{2}(z-{z}_{0})-\frac{2\omega ({k}^{2}+{l}^{2})}{k{C}_{R}}{\alpha}^{2}-1-\frac{\omega}{k{C}_{R}},$(3)

*if*$2dk{C}_{R}=-{\omega}^{2}{({k}^{2}+{l}^{2})}^{2}{\alpha}^{4}+{(\omega +k{C}_{k})}^{2}$, $\alpha \ne 0$, ${z}_{0}\in \mathbb{C}$.

- (III)
*The rational function solutions are*${w}_{R}(z)=\frac{12\omega ({k}^{2}+{l}^{2})}{k{C}_{R}{(z-{z}_{0})}^{2}}-1-\frac{\omega}{k{C}_{R}},$(4)

*if* $2dk{C}_{R}={(\omega +k{C}_{k})}^{2}$, ${z}_{0}\in \mathbb{C}$.

## 2 Preliminary lemmas and the complex method

In order to explain our complex method and give the proof of Theorem 1, we need some lemmas and results.

*Let*$k\in \mathbb{N}$,

*then any meromorphic solution*

*w*

*with at least one pole of the*

*kth order Briot*-

*Bouquet equation*,

*belongs to* *W*, *where* ${P}_{i}(w)$ *are polynomials in* *w* *with constant coefficients*.

where ${a}_{r}$ are constants, and *I* is a finite index set. The total degree of $P(w,{w}^{\prime},\dots ,{w}^{(m)})$ is defined by $degP(w,{w}^{\prime},\dots ,{w}^{(m)}):={max}_{r\in I}\{p(r)\}$.

where $b\ne 0$, *c* are constants, $n\in \mathbf{N}$.

**Definition 2**Let $p,q\in \mathbf{N}$. Suppose that Eq. (5) has a meromorphic solution

*w*with at least one pole; then we say that Eq. (5) satisfies the weak $\u3008p,q\u3009$ condition if substituting the Laurent series

*p*distinct Laurent singular parts in the form below:

*Let*$p,l,m,n\in \mathbf{N}$, $degP(w,{w}^{(m)})<n$.

*Suppose that the*

*mth order Briot*-

*Bouquet equation*

*satisfies the weak*$\u3008p,q\u3009$

*condition*;

*then all meromorphic solutions*

*w*

*belong to the class*

*W*.

*If for some values of parameters such a solution*

*w*

*exists*,

*then other meromorphic solutions form a one*-

*parametric family*$w(z-{z}_{0})$, ${z}_{0}\in \mathbf{C}$.

*Furthermore each elliptic solution with a pole at*$z=0$

*can be written as*

*where* ${c}_{-ij}$ *are given by Eq*. (6), ${B}_{i}^{2}=4{A}_{i}^{3}-{g}_{2}{A}_{i}-{g}_{3}$, *and* ${\sum}_{i=1}^{l}{c}_{-i1}=0$.

*Each rational function solution*$w:=R(z)$

*is of the form*

*with* *l* (≤*p*) *distinct poles of multiplicity* *q*.

*Each simply periodic solution is a rational function*$R(\xi )$

*of*$\xi ={e}^{\alpha z}$ ($\alpha \in \mathbf{C}$). $R(\xi )$

*has*

*l*(≤

*p*)

*distinct poles of multiplicity*

*q*,

*and it is of the form*

In order to give the representations of the elliptic solutions, we need some notation and results concerning the elliptic function [6].

where ${g}_{2}=60{s}_{4}$, ${g}_{3}=140{s}_{6}$ and $\mathrm{\Delta}({g}_{2},{g}_{3})\ne 0$.

we have ${e}_{1}=\mathrm{\wp}({\omega}_{1})$, ${e}_{2}=\mathrm{\wp}({\omega}_{2})$, ${e}_{3}=\mathrm{\wp}({\omega}_{1}+{\omega}_{2})$.

Inversely, given two complex numbers ${g}_{2}$ and ${g}_{3}$ such that $\mathrm{\Delta}({g}_{2},{g}_{3})\ne 0$, there exists a Weierstrass elliptic function $\mathrm{\wp}(z)$ with double periods $2{\omega}_{1}$, $2{\omega}_{2}$ such that the above holds.

*The Weierstrass elliptic functions*$\mathrm{\wp}(z):=\mathrm{\wp}(z,{g}_{2},{g}_{3})$

*have two successive degeneracies and in addition we have the following*.

- (I)
*Degeneracy to simply periodic functions*(*i*.*e*.,*rational functions of one exponential*${e}^{kz}$)*occurs according to*$\mathrm{\wp}(z,3{d}^{2},-{d}^{3})=2d-\frac{3d}{2}{coth}^{2}\sqrt{\frac{3d}{2}}z$(13)

*if one root*${e}_{j}$

*is double*($\mathrm{\Delta}({g}_{2},{g}_{3})=0$).

- (II)
*Degeneracy to rational functions of**z**occurs according to*$\mathrm{\wp}(z,0,0)=\frac{1}{{z}^{2}}$

*if one root*${e}_{j}$

*is triple*(${g}_{2}={g}_{3}=0$).

- (III)
*The addition formula holds according to*$\mathrm{\wp}(z-{z}_{0})=-\mathrm{\wp}(z)-\mathrm{\wp}({z}_{0})+\frac{1}{4}{\left[\frac{{\mathrm{\wp}}^{\prime}(z)+{\mathrm{\wp}}^{\prime}({z}_{0})}{\mathrm{\wp}(z)-\mathrm{\wp}({z}_{0})}\right]}^{2}.$(14)

*By the above lemmas*, *we can give a new method below*, *called*, *say*, *the complex method*, *to find exact solutions of some PDEs*.

Step 1. *Substituting the transform* $T:u(x,y,t)\to w(z),(x,y,t)\to z$ *into a given PDE gives a non*-*linear ordinary differential equation* (5) *or* (7).

Step 2. *Substitute Eq*. (6) *into Eq*. (5) *or* (7) *to determine whether the weak* $\u3008p,q\u3009$ *condition holds*.

Step 3. *By the indeterminate relations* (8)-(10) *we find the elliptic*, *rational*, *and simply periodic solutions* $w(z)$ *of Eq*. (5) *or* (7) *with pole at* $z=0$, *respectively*.

Step 4. *By Lemmas* 1 *and* 3 *we obtain all meromorphic solutions* $w(z-{z}_{0})$.

Step 5. *Substituting the inverse transform* ${T}^{-1}$ *into these meromorphic solutions* $w(z-{z}_{0})$, *we get all exact solutions* $u(x,t)$ *of the originally given PDE*.

## 3 Proof of Theorem 1

Substituting (6) into Eq. (1) we have $q=2$, $p=1$, ${c}_{-2}=\frac{12\omega ({k}^{2}+{l}^{2})}{k{C}_{R}}$, ${c}_{-1}=0$, ${c}_{0}=-1-\frac{\omega}{k{C}_{R}}$, ${c}_{1}=0$, ${c}_{2}=\frac{{(\omega +k{C}_{R})}^{2}-2dk{C}_{R}}{20\omega ({k}^{2}+{l}^{2})k{C}_{R}}$, ${c}_{3}=0$, ${c}_{4}$ is arbitrary.

Hence, Eq. (1) satisfies the weak $\u30081,2\u3009$ condition and is a second-order Briot-Bouquet differential equation. Obviously, Eq. (1) satisfies the dominant condition. So, by Lemma 3, we know that all meromorphic solutions of Eq. (1) belong to *W*. Now we will give the forms of all meromorphic solutions of Eq. (1).

where $2dk{C}_{R}={(\omega +k{C}_{R})}^{2}$, ${z}_{0}\in \mathbb{C}$.

where $2dk{C}_{R}=-{\omega}^{2}{({k}^{2}+{l}^{2})}^{2}{\alpha}^{4}+{(\omega +k{C}_{k})}^{2}$, $\alpha \ne 0$, ${z}_{0}\in \mathbb{C}$.

Here, $2dk{C}_{R}=-12{\omega}^{2}{({k}^{2}+{l}^{2})}^{2}{g}_{2}+{(\omega +k{C}_{k})}^{2}$, ${F}^{2}=4{E}^{3}-{g}_{2}E-{g}_{3}$, ${g}_{3}$, and *E* are arbitrary.

## 4 Computer simulations for new solutions

- (1)
Take $k=l=1$, $\omega ={C}_{R}=-1$, $d=2$, ${z}_{0}=0$ in ${w}_{R}(z)$.

- (2)
Take $k=l=1$, $\omega ={C}_{R}=-1$, $d=2$, ${z}_{0}=0$, $\alpha =1$ in ${w}_{s}(z)$.

## 5 Conclusions

The complex method is a very important tool in finding the traveling wave exact solutions of non-linear evolution equations such as the Petviashvili equation. In this paper, we employ the complex method to obtain all meromorphic exact solutions of the complex variant Eq. (1); then we find all traveling wave exact solutions of the Petviashvili equation.The idea introduced in this paper can be applied to other non-linear evolution equations. Our result shows that the complex method is simpler than other methods.

## Declarations

### Acknowledgements

This work is supported by the NSF of China (11271090) and NSF of Guangdong Province (S2012010010121). Also this work was supported by the Visiting Scholar Program of the Chern Institute of Mathematics at Nankai University where the authors worked as visiting scholars. The authors would like to express their hearty thanks to the Chern Institute of Mathematics providing very comfortable research environments to them. The authors finally wish to thank Professor Robert Conte for supplying his useful reprints and suggestions. The authors wish to thank the referees and editors for their very helpful comments and useful suggestions.

## Authors’ Affiliations

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## Copyright

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