# Blow-up phenomena and stability of solitary waves for a generalized Dullin-Gottwald-Holm equation

- Nurhan Dündar
^{1}Email author and - Necat Polat
^{1}

**2013**:226

https://doi.org/10.1186/1687-2770-2013-226

© Dündar and Polat; licensee Springer. 2013

**Received: **21 May 2013

**Accepted: **3 September 2013

**Published: **7 November 2013

## Abstract

In this work, we consider the Cauchy problem of the generalized Dullin-Gottwald-Holm equation. We establish a blow-up result for the generalized Dullin-Gottwald-Holm equation. In addition to this, we investigate the stability of solitary wave solutions of the equation.

## Keywords

## 1 Introduction

is, in the dimensionless space-time variable $(x,t)$, a model for unidirectional shallow water waves over a flat bottom. Here, $y=u-{\alpha}^{2}{u}_{xx}$ is a momentum variable, the constants ${\alpha}^{2}$ and $\frac{\gamma}{{c}_{0}}$ are squares of length scales, and ${c}_{0}=\sqrt{gh}>0$ (where ${c}_{0}=2\omega $) is the linear wave speed for undisturbed water at rest at spatial infinity, where *h* is the mean fluid depth and *g* is the gravitational constant. Dullin, Gottwald and Holm derived Eq. (1) by using asymptotic expansions directly in the Hamiltonian for Euler’s equations in the shallow water regime in [1]. Eq. (1) was shown to be bi-Hamiltonian and to have a Lax pair formulation. The Dullin-Gottwald-Holm equation (we call it DGH equation for short) is an integrable system via the inverse scattering transform (IST) method and contains both the Korteweg-de Vries (KdV) and Camassa-Holm (CH) equations [2] as limiting cases.

Recently, the DGH equation has been studied by many authors. Tian *et al.* [3] studied the well-posedness of the Cauchy problem and the scattering problem for the DGH equation. In [4], Hakkaev proved the orbital stability of the peaked solitary waves for the DGH equation by using the method in [5]. It was shown that the DGH equation has global solutions and blow-up solutions in [6–9].

Eq. (3) was studied in [10, 11]. In [10], Lu *et al.* studied the local well-posedness of the Cauchy problem for Eq. (3). In [11], the conservation laws for the generalized DGH equation were derived.

was studied in [12]. For $h(u)=2\omega u+\frac{3}{2}{u}^{2}$ and $g(u)=u$, Eq. (4) becomes DGH Eq. (2).

One of the aims of this paper is to give the precise blow-up scenario and to show that Eq. (4) has blow-up solutions for $g(u)=u$. In addition to this, we investigate the stability of solitary wave solutions of Eq. (4) with $h(u)=2\omega u+\frac{p+2}{2}{u}^{p+1}$ and $g(u)={u}^{p}$.

The remainder of the paper is organized as follows. In Section 2, we give our basic notation and recall some required results. In Section 3, we investigate blow-up of solutions for Eq. (4). In Section 4, we prove the stability of solitary wave solutions with the help of the orbital stability theory [13].

## 2 Preliminaries

and $(\cdot ,\cdot )$ for its inner product. For the sake of simplicity, we employ the same symbol *c* for different positive constants.

Some useful lemmas are as follows.

**Lemma 2.1** [14]

*Assume that*$s>0$.

*Then we have*

*Here* *c* *is a constant depending only on* *s*.

**Lemma 2.2** [15]

*Assume that*$F\in {C}^{m+2}(R,R)$

*with*$F(0)=0$.

*Then*,

*for every*$\frac{1}{2}<s\le m$,

*we have that*

*where* $\tilde{F}$ *is a monotone increasing function depending only on* *F* *and* *s*.

**Lemma 2.3** [14]

*Assume that*$s>0$.

*Then*${H}^{s}\cap {L}^{\mathrm{\infty}}$

*is an algebra*.

*Moreover*,

*where* *c* *is a constant depending only on* *s*.

**Lemma 2.4** [16]

*Let*$T>0$

*and*$u\in {C}^{1}([0,T);{H}^{2})$.

*Then*,

*for every*$t\in [0,T)$,

*there exists at least one pair of points*$\xi (t),\zeta (t)\in R$

*such that*

*and*$m(t)$, $M(t)$

*are absolutely continuous in*$(0,T)$.

*Moreover*,

**Theorem 2.1** [12]

*Assume that*$h,g\in {C}^{m+3}(R,R)$, $m\ge 2$

*and*$h(0)=g(0)=0$.

*Given*${u}_{0}\in {H}^{s}$, $\frac{3}{2}<s\le m$,

*there exists a maximal*$T=T({u}_{0})>0$

*and a unique solution*

*u*

*to Eq*. (4)

*such that*

*Moreover*,

*the solution depends continuously on the initial data*,

*i*.

*e*.,

*the mapping*

*is continuous*.

**Lemma 2.5** [12]

*Let*$u(t,x)$

*be a solution of Eq*. (4).

*Then the functionals*

*are constant with respect to* *t*, *where* ${H}^{\prime}(s)=h(s)$.

## 3 Blow-up phenomena

In this section, we discuss the blow-up phenomena of Eq. (4) with $g(u)=u$. For Eq. (4), which describes shallow water waves, the blow-up occurs only in the form of wave-breaking, *i.e.* the solution remains bounded but its slope becomes unbounded in finite time.

where $k(u)=(h(u)+\frac{{\alpha}^{2}}{2}{u}_{x}^{2}-\frac{1}{2}{u}^{2}+\frac{\gamma}{{\alpha}^{2}}u)$.

We now prove the following result.

**Theorem 3.1**

*Let*$h\in {C}^{m+3}(R,R)$, $m\ge 2$,

*and*${u}_{0}\in {H}^{r}$, $\frac{3}{2}<r\le m$.

*If*

*T*

*is the existence time of the corresponding solution of initial data*${u}_{0}$,

*then the*${H}^{r}$-

*norm of*$u(t,x)$

*to Eq*. (4) (

*or*(6))

*blows up on*$[0,T)$

*if and only if*

*Proof*Let $u(t,x)$ be the solution of Eq. (4) with the initial data ${u}_{0}\in {H}^{r}$, $\frac{3}{2}<r\le m$, which is guaranteed by Theorem 2.1. If

by Sobolev’s embedding theorem, we obtain that the solution $u(t,x)$ will blow up in finite time.

*R*, we obtain

This completes the proof of Theorem 3.1. □

We have the following blow-up scenario for Eq. (4).

**Theorem 3.2**

*Assume that*$h\in {C}^{m+3}(R,R)$, $m\ge 3$.

*Given*${u}_{0}\in {H}^{r}$, $3\le r\le m$,

*the solution*$u=u({u}_{0},\cdot )$

*of Eq*. (4)

*is uniformly bounded*.

*Blow*-

*up in finite time*$T<+\mathrm{\infty}$

*occurs if and only if*

*Proof*$E(u)=\frac{1}{2}{\int}_{R}({u}^{2}+{\alpha}^{2}{u}_{x}^{2})\phantom{\rule{0.2em}{0ex}}dx$ is an invariant for Eq. (4). According to the inequalities

the invariance of $E(u)$ ensures that the solution *u* is uniformly bounded as long as they exist.

If the slope of the solution $u(t,x)$ becomes unbounded from below in finite time, then by Theorem 2.1 and Sobolev’s embedding theorem, we can see that the solution $u(t,x)$ blows up in finite time.

*x*, in view of the identity ${\partial}_{x}^{2}(p\ast f)=\frac{1}{{\alpha}^{2}}(p\ast f-f)$, we have

By Theorem 3.1 and the above inequality, we have that if the slope of the solution is bounded from below in finite time, then the solution will not blow up in finite time. □

Next, we present the following blow-up result.

**Theorem 3.3**

*Assume that*$h\in {C}^{m+3}(R,R)$, $m\ge 3$.

*Given*$\alpha >0$, ${u}_{0}\in {H}^{r}$, $3\le r\le m$,

*assume that we can find*${x}_{0}\in R$

*with*

*where*

*Then the corresponding solution to Eq*. (4)

*for*$g(u)=u$

*blows up in finite time*.

*Moreover*,

*the maximal time of existence*

*T*

*satisfies the inequality*

*Proof*Now define $m(t)={inf}_{x\in R}[{u}_{x}(t,x)]={u}_{x}(t,\xi (t))$ by Lemma 2.4, and let $\xi (t)\in R$ be a point where this infimum is attained. From Eq. (10), we have

*T*is obtained immediately, namely

The conclusion is reached by letting $c\to -m(0)$. □

## 4 Stability of solitary waves

The appropriate notion of stability for the solitary waves here is orbital stability: a wave starting close to a solitary wave should stay close, as long as it exists, to some translate of the solitary wave. The orbit of a solitary wave is the set of all its translates [17].

We define the orbit $\phi (\cdot -\eta )=\{\tau (\eta )\phi (\cdot ):\eta \in R\}$, where *τ* is a one-parameter group of unitary operators on ${H}^{2}$ defined by $\tau (s)u(\cdot )=u(\cdot -s)$, $s\in R$, $u\in {H}^{2}$. They may be interpreted physically as ‘solitary waves’ or ‘bound states.’

**Definition**The

*φ*-orbit is stable if for all $\epsilon >0$, there exists $\delta >0$ with the following property. If ${\parallel u(0,\cdot )-\phi (\cdot )\parallel}_{{H}^{2}}<\delta $ and

*u*is a solution of Eq. (12) in some interval $[0,T)$ with $u(0,\cdot )={u}_{0}$, then

*u*can be continued to a solution in $0\le t<\mathrm{\infty}$ and

Otherwise, the *φ*-orbit is called unstable.

Definition can be seen in reference [13].

*σ*, we obtain

*E*and

*F*,

*E*and

*F*, respectively. To study the orbital stability of the solitary waves of Eq. (12), we need the operator ${H}_{c}$ and the function $d(c)=cE(\phi )-F(\phi )$. The linearized operator ${H}_{c}$ around

*φ*is defined by

**Lemma 4.1** *For each* $c\in (2\omega ,\mathrm{\infty})$, ${H}_{c}=c{E}^{\u2033}(\phi )-{F}^{\u2033}(\phi )$ *has a unique simple negative eigenvalue*, *zero is a simple eigenvalue*, *and the rest of its spectrum is bounded away from zero*.

*Proof*For any $u,v\in {H}^{2}$, we have ${\int}_{R}{u}_{xx}v\phantom{\rule{0.2em}{0ex}}dx={\int}_{R}u{v}_{xx}\phantom{\rule{0.2em}{0ex}}dx$ and

Differentiating (13) with respect to *σ*, we find that the right-hand side of Eq. (15) equals zero, that is, ${H}_{c}{\phi}^{\prime}=0$. The behavior of the function *φ* tells us that ${\phi}^{\prime}$ has exactly one zero. So, the zero eigenvalue of the operator ${H}_{c}$ is simple, and by using the Sturm-Liouville theorem, we have that ${H}_{c}$ only has a negative eigenvalue. Using Weyl’s theorem, the essential spectrum of ${H}_{c}$ belongs to $[\frac{c-2\omega}{{\alpha}^{2}},+\mathrm{\infty})$. That completes the proof. □

As for the above results, it is known [13] that stability would be ensured by the convexity of the scalar function $d(c)=cE(\phi )-F(\phi )$, $c>2\omega $. So, we obtain the following theorem under the condition $2\omega {\alpha}^{2}+\gamma =0$.

**Theorem 4.1** *Suppose that* $\phi (\cdot -\eta )$ *is a solitary wave solution for* $c>2\omega $, *if* ${d}^{\u2033}(c)>0$, *then* $\phi (\cdot -\eta )$ *is stable*.

*ϑ*such that

*φ*satisfies

Multiplying solitary wave equation (13) by *φ* and integrating the resulting equation gives $I(\phi )=\frac{(p+2)}{2}K(\phi )$.

We say that ${\psi}_{k}$ is a minimizing sequence if for some $\lambda >0$, ${lim}_{k\to \mathrm{\infty}}I({\psi}_{k})={M}_{\lambda}$ and ${lim}_{k\to \mathrm{\infty}}K({\psi}_{k})=\lambda $.

**Theorem 4.2** *Let* $\{{\psi}_{k}\}$ *be a minimizing sequence for some* $\lambda >0$. *If* $c>2\omega $, *then there exists a subsequence* $\{{\psi}_{{k}_{j}}\}$, *scalars* ${y}_{j}\in R$ *and* $\psi \in {H}^{2}$ *such that* ${\psi}_{{k}_{j}}\to \psi $ *in* ${H}^{2}$. *The function* *ψ* *achieves the minimum* $I(\psi )={M}_{\lambda}$ *subject to the constraint* $K(\psi )=\lambda $.

*Proof* The result is an application of the concentration compactness lemma of Lions [19, 20]. Proof is similar to [21–23] according to the definition of $I(u)$ and $K(u)$.

In this section, we show that the stability of solitary wave solutions is determined by the convexity of the function $d(c)$.

We state the basic properties of the function *d*.

Therefore, *d* is well defined, and we may deduce its properties by examining the function $m(\omega ,c)$. □

**Lemma 4.2** *For fixed* $\omega \in R$, $m(\omega ,c)$ *is monotonically increasing in* *c*.

*Proof*We assume that ${\phi}_{{c}_{1}}$, ${\phi}_{{c}_{2}}$are solutions of Eq. (13) corresponding to $c={c}_{1}$, $c={c}_{2}$, respectively. Without loss of generality, let be ${c}_{1}<{c}_{2}$, then we have

This shows that *m* is monotonically increasing in *c*, so that by (18), *d* must be monotonically increasing as well.

*c*that

□

The following lemma is helpful in order to prove the stability of solitary waves.

**Lemma 4.3**

*If*${d}^{\u2033}(c)>0$,

*then there exists*$\epsilon >0$

*such that for any*$u\in {U}_{\omega ,c;\epsilon}$,

*we have*

*Proof*By using ${d}^{\prime}(c)=E(\phi )$ and Taylor’s formula, we have the expansion

*c*. Using the continuity of $c(u)$ and choosing

*ε*sufficiently small, we get that

□

*Proof of Theorem 4.1*Suppose that $\phi (\cdot -\eta )$ is unstable. Then there exists ${\epsilon}_{0}$ and a sequence of initial data ${u}_{k}(0)$ satisfying

*t*, and there exist times ${t}_{k}$ such that

*E*and

*F*are invariants of (12) so that

This implies that $c({u}_{k}({t}_{k}))\to c$, as $k\to \mathrm{\infty}$, since ${u}_{k}({t}_{k})$ is uniformly bounded for *k*.

*d*implies that

From (22) and (23), ${u}_{k}({t}_{k})$ is a minimizing sequence for the pair *I*, *K* and thus by Theorem 4.2 has a subsequence, named ${u}_{{k}_{j}}({t}_{{k}_{j}})$, that converges in ${H}^{2}$ to some *φ*. This contradicts (20). The proof is completed. □

## Declarations

### Acknowledgements

This work was supported by Grant No. 13-FF-46 of the Dicle University of Scientific Research Projects Coordination (DUBAP), Diyarbakir, Turkey.

## Authors’ Affiliations

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