### Abstract

In this article, we establish some blow-up results for a modified two-component Camassa-Holm
system in Sobolev spaces. We also obtain the existence of the weak solutions of this
system in *H ^{s }*×

*H*

^{s-1},

*s*> 5/2.

##### Keywords:

the modified two-component Camassa-Holm equations; blow up; weak solutions.### 1. Introduction

The well-known two-component Camassa-Holm equations [1]

where *m *= *u *- *u _{xx }*and

*σ*= ± 1. Constantin and Ivanov [2] derived this system in the context of shallow water theory.

*u*can be interpreted as the horizontal fluid velocity and

*ρ*is related to the water elevation in the first approximation [2,3]. They showed that while small initial data develop into global solutions, for some initial data wave breaking occurs. They also discussed the solitary wave solutions. In Vlasov plasma models, system (1.1) describes the closure of the kinetic moments of the single-particle probability distribution for geodesic motion on the simplectomorphisms. While in the large-deformation diffeomorphic approach to image matching, system (1.1) is summoned in a type of matching procedure called metamorphosis (see [4] and the references therein). This system appeared originally in [5]. Based on the deformation of bi-Hamiltonian structure of the hydrodynamic type, Chen et al. [6] obtained system (1.1) when

*σ*= -1. They show that it has the peakon and multilink solitons, and is integrable in the sense that it has Lax-pair. The mathematical properties of system (1.1) have been studied further in many articles, see, e.g., [7-15]. In [4], Holm and Ivanov generalized the Lax-pair formulation of system (1.1) to produce an integrable multi-component family, CH(

*n, k*), of equations with

*n*components and 1 ≤ |

*k*| ≤

*n*velocities. They determined their Lie-Poisson Hamiltonian structures and gave numerical examples of their soliton solution behavior. Recently, a new global existence result and several new blow-up results of strong solutions for the Cauchy problem of Equation (1.1) with

*σ*= 1 were obtained in [8]. Gui and Liu [14] established the local well posedness for the two-component Camassa-Holm system in a range of the Besov spaces. Chen and Liu [16] discussed the wave-breaking phenomenon of a generalized two-component Camassa-Holm system, and determined the exact blow-up rate of such solutions. The existence and uniqueness of global weak solutions to Equation (1.1) have also been discussed by Guan and Yin [17].

In this article, we consider a two-component generalization of Equation (1.1), that is

with initial data

where *m *≥ 1. It can be reduced to (1.1) as *m *= 1.

The purpose of this article is to study the well posedness, local weak solution, and blow-up for Cauchy problem (1.2) and (1.3). System (1.2) also conserves conservation laws. Our starting point is to obtain the local well posedness by using Kato's theory, Next, we derive some blow-up results of the solutions by the following transport equation,

which is a crucial ingredient to obtain the blow-up phenomenon. Last, by using the
conserves from laws and the contraction mapping theorem, we obtain the existence of
weak solutions of Cauchy problem (1.2) and (1.3). These methods are similar to that
was used in [18]. However, because of the asymmetry and the high strength of the nonlinearity of Equation
(1.3), it is more difficult to estimate the norm of *u, ρ, u _{x}, ρ_{x }*in Sobolev space. In addition, also we get Equation (5.10) which is different with
that in [18]. As for the blow-up phenomenon, we get some new results of (1.2) and (1.3).

Guan and Yin [17,19] got the global weak solutions for two-component Camassa-Holm shallow water system;
they first obtained approximate solutions for the system, then they prove the compactness
of these solutions, and at last they got the global weak solutions. Using the same
way, Liu and Yin [20] also got global weak solutions for a periodic two-component *μ*-Hunter-Saxton system. However, in this article, we add high-order perturbation terms
in this system, and by using the conserves laws and the contraction mapping theorem,
we obtain the existence of weak solutions.

The remainder of this article is organized as follows. Section 2 is the preliminary. In Section 3, the local well posedness for strong solution of Cauchy problem (1.2) and (1.3) is established by Kato's theory. In Section 4, by transport equation, some blow-up results of the solutions of Cauchy problem (1.2) and (1.3) are obtained. The proof of existence of local weak solution is carried out in Section 5.

### 2. Blow-up

**Lemma 2.1**: Given
*s *> 5/2, then there exists a maximal

Moreover, the solution depends continuously on the initial data, i.e., the mapping

is continuous.

The proof is similar with Theorem 4.1 in [21].

Let

Consider the following initial value problem,

where *u *is the first component of the solution *z *to Equation (1.2).

To prove the blow-up result, we need the following lemma.

**Lemma 2.2**: Let *z*_{0}*∈ H ^{s }*×

*H*

^{s-1}, (

*s*> 5/2), and let

*T*> 0 be the maximal existence time of the corresponding solution

*z*to Equation (2.1), then we have

** Proof**. Differentiating the left-hand side of Equation (2.3) with respect

*t*. It follows from (2.1) and (2.2), that

This completes the proof of this lemma.

**Theorem 2.1**: Let
*s *> 5/2), and *T *be the maximal time of the solution *z *to Equation (1.2) with the initial data *z*_{0}. Assume that there exists *x*_{0 }∈ *R *such that

Then, *T *is finite and the slope of *u *tends to negative infinity as *t *goes to *T *while *u *is uniformly bounded on [0, *T*).

** Proof**. Let

*z*

_{0}, and

*T*be the maximal time of

*z*, and let

From (2.1) and (2.2), we have

Differentiating the first equation in (2.1) with respect *x*, we have

Note that
*γ*(*t*) = 0, ∀*t *∈[0, *T*).

Thus

Since

Note that

Since

With the inequality above, we get

Since

This completes the proof of the theorem.

### 3. Local weak solution

**Definition 3.1**: ([22]) Let (*u*_{0}, *ρ*_{0}) ∈ *H*^{1}(*R*) × *H*^{1}(*R*). If (*u, ρ*) belongs to

for all
*T*) × *R*) × ([0,*T*) × *R*) of smooth functions on *R*^{2 }× *R*^{2 }with compact support contained in ((-*T, T*) × *R*) × ((-*T, T*) × *R*). Then, *z *is called a weak solution to Equation (1.6). If *z *is a weak solution on [0, *T*) × [0, *T*) for every *T *> 0, then it is called global weak solution to Equation (1.6).

In this section, we discuss the existence of weak solution of Cauchy problem (1.2) and (1.3). To this purpose, we consider the following Cauchy problem:

where *ε *is a constant satisfying 0 < *ε *< 1/4. Note that when *ε *= 0, system (3.1) and (3.2) is just the system (1.2) and (1.3).

For any 0 < *ε *< 1/4 and *s *≥ 1, the integral operators

and

define two bounded linear operator in the indicated Sobolev spaces.

To prove the existence of solutions to the problem (3.1) and (3.2), we apply the two
operators above to both sides of (3.1) and then integrate the resulting equations
with regard to *t*. This leads to the following equations.

A standard application of the contraction mapping theorem leads to the following existence result.

**Theorem 3.1: **For each initial data *u*_{0 }∈ *H ^{s }*(

*s*≥ 1),

*ρ*

_{0 }∈

*H*

^{s-1}(

*s*≥ 2), there exists a

*T*> 0 depending only on the norm of

*m*such that there exists a unique solution (

*u, ρ*) ∈

*C*([0,

*T*];

*H*) ×

^{s}*C*([0,

*T*];

*H*

^{s-1}) of system (3.1) and (3.2) in the sense of distribution. If

*u*

_{0 }∈

*H*(

^{s }*s*≥ 2),

*ρ*

_{0 }∈

*H*

^{s-1}(

*s*≥ 3), the solution (

*u, ρ*) ∈

*C*([0,∞];

*H*) ×

^{s}*C*([0, ∞];

*H*

^{s-1}) exists for all time, in particular, when

*u*

_{0 }∈

*H*(

^{s }*s*≥ 4),

*ρ*

_{0 }∈

*H*

^{s-1}(

*s*≥ 5), the corresponding solution is a classical globally defined solution of (3.1) and (3.2).

The global existence result follows from the conservation law

admitted by (3.1) in its integral form.

**Theorem 3.2: **Suppose that for some *s *≥ 4, the function pair *u*(*x, t*) and *ρ*(*x, t*) in the solution of Equation (3.1) corresponding to the initial data *u*_{0 }∈ *H ^{s }*(

*s*≥ 4);

*ρ*

_{0 }∈

*H*

^{s-1}(

*s*≥ 5), then the following inequalities hold:

For any real number *q *∈ (1, *s*] (*s *≥ 5), there exists a constant *c *depending only on *q, m*, such that

For any *q *∈ (1, *s*-1] (*s *≥ 4), there exists a constant *c *such that

And for any *q *∈ (1, *s*-2] (*s *≥ 5), there exists a constant *c *such that

** Proof**. It is obvious that (3.3) holds. In order to prove (5.4), let

For any *q *∈ (1, *s*] (*s *≥ 5), applying (Λ* ^{q }u*)Λ

*to the both sides of the first equation of Equation (3.7), respectively, and integrating with regard to*

^{q }*x*, we obtain

By using Sobolev embedding theorems, we have

where we have used lemma in [23] with *r *= *q *-2 > 0. Also

where we have used Lemma in [24] with *r *= *q *-1 > 0.

Then, we get

For any *q *∈ (1, *s*-1] (*s *≥ 5), applying (Λ* ^{q-1 }ρ*) Λ

*to the both sides of the second equation of Equation (3.7), respectively, then we obtain*

^{q-1 }

Summing up (3.8) and (3.9), we get

For any *q *∈ (1, *s*-1] (*s *≥ 4), applying (Λ* ^{q }u_{t}*)Λ

*to the both sides of the first equation of Equation (3.7), respectively, and integrating with regard to*

^{q }*x*, we obtain that

and

where we have used lemma in [24] with *r *= *q *-1 > 0. Then, we get

For any *q *∈ (1, *s*-2] (*s *≥ 5), applying (Λ^{q-1 }*ρ _{t}*)Λ

^{q-1 }to the both sides of the second equation of Equation (5.7), respectively, then we obtain

This complete the proof of the theorem.

Suppose *u*_{0 }∈ *H ^{s }*(

*s*≥ 1),

*ρ*

_{0 }∈

*H*

^{s-1}(

*s*≥ 2), and let

*u*

_{ε0},

*ρ*

_{ε0 }be the convolution

*u*

_{ε0 }=

*φ**

_{ε}*u*

_{0},

*ρ*

_{ε0 }=

*φ**

_{ε}*ρ*

_{0}, where

*φ*satisfies

*ξ*∈ (-1,1). Then, it follows from Theorem 3.1 that for each

*ε*with 0 <

*ε*<1/4, the Cauchy problem

has a unique solution *u _{ε }*(

*t, x*) ∈

*C*

^{∞}([0,∞);

*H*

^{∞ }and

*ρ*(

_{ε }*t, x*) ∈

*C*

^{∞}([0,∞);

*H*

^{∞}. We first demonstrate the properties of the initial data

*u*

_{ε0},

*ρ*

_{ε0 }in the following lemma. The proof is similar to Lemma 5 in [25].

**Lemma 3.1: **Under the above assumptions, there hold

for any *ε *with
*c *is a constant independent of *ε*. The proof is similar to Lemma 5 in [25].

**Theorem 3.3**: Suppose that *u*_{0}(*x*) ∈ *H ^{s}*(

*R*),

*s*∈ [1, 3/2];

*ρ*

_{0}(

*x*) ∈

*H*

^{s-1}(

*R*),

*s*-1 ∈ [1, 3/2] such that
*u*_{ε0 }= *φ _{ε}**

*u*

_{0},

*ρ*

_{ε0 }=

*φ**

_{ε}*ρ*

_{0}, be defined the same as above. Then, there exist constants

*T*> 0 and

*c*> 0 independent of

*ε*such that the corresponding solution

*u*of (3.10) satisfy the inequalities

_{ε}, ρ_{ε }*t*∈ [0,

*T*).

** Proof**. Use Equation (3.7) with

*u*=

*u*=

_{ε}, ρ*ρ*. Differentiating with respect to

_{ε}*x*on both sides of the first equation in Equation (3.7). Note that

Let *n *> 0 be an integer. Then, multiplying the above equation by (*u _{x}*)

^{2n+1 }to integrate with respect to

*x*, we get

where

It follows from *Hölder *inequality that

Note that
*p*→∞ for any *f *∈ *L*^{∞}∩*L*^{2}. Integrating the above inequality over *R *with respect to *t*, and taking the limitation as *n*→∞, we have

It follows from (3.3) that

For any given *r *∈ (1/2,1), we have

Then from (3.4), we have

and

Thus,

and

Then, we get

It follows that

*c *is a constant depends on Λ^{-2 }and *m*.

Also, we can obtain

where

It follows from the contraction mapping theorem that there exists a constant *T *> 0 such that the equation

has a unique solution *f*(*t*) ∈ *C *[0,*T*]. Theorem II in Section I.1 in [26] shows that
*t *∈ [0,*T*] which leads to the conclusion of this theorem.

Let *u *= *u _{ε}, ρ *=

*ρ*, with (3.4) used

_{ε}

where

where *q *∈ (0, s], *r *∈ (0, s-1], *t *∈ [0,*T*].

Then, it follows from Aubin's compactness theorem [27] that there exist subsequences of {*u _{ε}*}, {

*ρ*} denoted by

_{ε}*u*(

*t, x*)∈

*L*

^{2}([0,

*T*];

*H*),

^{s}*ρ*(

*t, x*)∈

*L*

^{2}([0,

*T*];

*H*

^{s-1}), respectively, and

*u*(

_{t}*t, x*)∈

*L*

^{2}([0,

*T*];

*H*

^{s-1}),

*ρ*(

_{t}*t, x*)∈

*L*

^{2}([0,

*T*];

*H*

^{s-2}), respectively. Because

*u*(

*t, x*)∈

*L*

^{2}([0,

*T*];

*H*),

^{s}*f*∈ (

*L*

^{2}([0,

*T*];

*H*))* =

^{s}*L*

^{2}([0,

*T*];

*H*) when

^{s}*n*→ ∞. Applying Riesz lemma, we conclude that there exists

Since
*n *→ ∞, we have
*R *> 0,
*u *∈ *L*^{2}([0,*T*]; *H ^{q}*(-

*R, R*)) for any

*q*∈ [0,

*s*-1); and