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The general meromorphic solutions of the Petviashvili equation
Advances in Difference Equations volume 2014, Article number: 24 (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 nonlinear 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.
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.
The Petviashvili equation [1, 2] is
where
are twodimensional Laplace and Jacobian operators, respectively, {C}_{R} is the linear zerodimensional 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.
Substituting the traveling wave transformation
into the Petviashvili equation gives a nonlinear ordinary differential equation
and integrating it yields the auxiliary ordinary differential equation
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 BriotBouquet equation,
belongs to W, where {P}_{i}(w) are polynomials in w with constant coefficients.
Set m\in \mathbf{N}:=\{1,2,3,\dots \}, {r}_{j}\in {\mathbf{N}}_{0}=\mathbf{N}\cup \{0\}, r=({r}_{0},{r}_{1},\dots ,{r}_{m}), j=0,1,\dots ,m. Define
p(r):={r}_{0}+{r}_{1}+\cdots +{r}_{m} is called the degree of {M}_{r}[w]. The differential polynomial P(w,{w}^{\prime},\dots ,{w}^{(m)}) is defined as follows:
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)\}.
We will consider the following complex ordinary differential equations:
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
into Eq. (5) we can determine 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 BriotBouquet 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 oneparametric 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].
Let {\omega}_{1}, {\omega}_{2} be two given complex numbers such that Im\frac{{\omega}_{1}}{{\omega}_{2}}>0, let L=L[2{\omega}_{1},2{\omega}_{2}] be discrete subset L[2{\omega}_{1},2{\omega}_{2}]=\{\omega \mid \omega =2n{\omega}_{1}+2m{\omega}_{2},n,m\in \mathbf{Z}\}, which is isomorphic to \mathbf{Z}\times \mathbf{Z}. The discriminant is \mathrm{\Delta}=\mathrm{\Delta}({c}_{1},{c}_{2}):={c}_{1}^{3}27{c}_{2}^{2} and
The Weierstrass elliptic function \mathrm{\wp}(z):=\mathrm{\wp}(z,{g}_{2},{g}_{3}) is a meromorphic function with double periods 2{\omega}_{1}, 2{\omega}_{2}, and satisfying the equation
where {g}_{2}=60{s}_{4}, {g}_{3}=140{s}_{6} and \mathrm{\Delta}({g}_{2},{g}_{3})\ne 0.
On changing Eq. (11) to the form
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 nonlinear 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 secondorder BriotBouquet 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).
By Eq. (9), we infer that the indeterminate rational solutions of Eq. (1) with pole at z=0 have the form of
Substituting {R}_{1}(z) into Eq. (1), we get
Here 2dk{C}_{R}={(\omega +k{C}_{R})}^{2}. Thus for all rational solutions of Eq. (1)
where 2dk{C}_{R}={(\omega +k{C}_{R})}^{2}, {z}_{0}\in \mathbb{C}.
In order to have simply periodic solutions, set \xi ={e}^{\alpha z}, put w=R(\xi ) into Eq. (1); then
Substituting
into Eq. (16), we obtain
Here 2dk{C}_{R}={(\omega +k{C}_{R})}^{2}{\omega}^{2}{({k}^{2}{l}^{2})}^{2}{\alpha}^{2}. Substituting \xi ={e}^{\alpha z} into the above relation, and then we get simply periodic solutions of Eq. (1) with pole at z=0:
So all simply periodic solutions of Eq. (1) are obtained by
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}.
From Eq. (8) of Lemma 3, we have indeterminate relations of the elliptic solutions of Eq. (1) with pole at z=0,
Putting {w}_{d0}(z) into Eq. (1), we obtain
Here 42dk{C}_{R}=12{\omega}^{2}{({k}^{2}+{l}^{2})}^{2}{g}_{2}+{(\omega +k{C}_{k})}^{2}. Therefore, for all elliptic solutions of Eq. (1)
where {z}_{0}\in \mathbb{C}. Making use of the addition formula of Lemma 4, we rewrite it in the form
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
In this section, we give some computer simulations to illustrate our main results.Here we take the new rational solutions {w}_{R}(z) and simply periodic solutions {w}_{s}(z) to further analyze their properties by Figures 1 and 2.

(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 nonlinear 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 nonlinear evolution equations. Our result shows that the complex method is simpler than other methods.
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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.
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ZH and WY carried out the design of the study and performed the analysis. JL participated in its design and coordination. All authors read and approved the final manuscript.
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Huang, Z., Yuan, W. & Lai, J. The general meromorphic solutions of the Petviashvili equation. Adv Differ Equ 2014, 24 (2014). https://doi.org/10.1186/16871847201424
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DOI: https://doi.org/10.1186/16871847201424
Keywords
 Petviashvili equation
 exact solution
 meromorphic function
 elliptic function