The existence of global weak solutions to the Cauchy problem for a weakly dissipative Camassa-Holm equation is established in the space under the assumption that the initial value only belongs to the space . The limit of viscous approximations, a one-sided super bound estimate and a space-time higher-norm estimate for the equation are established to prove the existence of the global weak solution.
MSC: 35G25, 35L05.
Keywords:global weak solution; Camassa-Holm type equation; existence
In this work, we investigate the Cauchy problem for the nonlinear model
where , , is a polynomial with order n, N and m are nonnegative integers. When , , , Eq. (1) is the standard Camassa-Holm equation [1-3]. In fact, the nonlinear term can be regarded as a weakly dissipative term for the Camassa-Holm model (see [4,5]). Here we coin (1) a weakly dissipative Camassa-Holm equation.
To link with previous works, we review several works on global weak solutions for the Camassa-Holm and Degasperis-Procesi equations. The existence and uniqueness results for global weak solutions of the standard Camassa-Holm equation have been proved by Constantin and Escher , Constantin and Molinet  and Danchin [8,9] under the sign condition imposing on the initial value. Xin and Zhang  established the global existence of a weak solution for the Camassa-Holm equation in the energy space without imposing the sign conditions on the initial value, and the uniqueness of the weak solution was obtained under certain conditions on the solution . Under the sign condition for the initial value, Yin and Lai  proved the existence and uniqueness results of a global weak solution for a nonlinear shallow water equation, which includes the famous Camassa-Holm and Degasperis-Procesi equations as special cases. Lai and Wu  obtained the existence of a local weak solution for Eq. (1) in the lower-order Sobolev space with . For other meaningful methods to handle the problems relating to dynamic properties of the Camassa-Holm equation and other partial differential equations, the reader is referred to [14-19]. Coclite et al. used the analysis presented in [10,11] and investigated global weak solutions for a generalized hyperelastic-rod wave equation (or a generalized Camassa-Holm equation), namely, , in Eq. (1). The existence of a strongly continuous semigroup of global weak solutions for the generalized hyperelastic-rod equation with any initial value in the space was established in . Up to now, the existence result of the global weak solution for the weakly dissipative Camassa-Holm equation (1) has not been found in the literature. This constitutes the motivation of this work.
The objective of this work is to study the existence of global weak solutions for the Eq. (1) in the space under the assumption . The key elements in our analysis include some new a priori one-sided upper bound and space-time higher-norm estimates on the first-order derivatives of the solution. Also, the limit of viscous approximations for the equation is used to establish the existence of the global weak solution. Here we should mention that the approaches used in this work come from Xin and Zhang  and Coclite et al..
The rest of this paper is as follows. The main result is given in Section 2. In Section 3, we present a viscous problem of Eq. (1) and give a corresponding well-posedness result. An upper bound, a higher integrability estimate and some basic compactness properties for the viscous approximations are also established in Section 3. Strong compactness of the derivative of the viscous approximations is obtained in Section 4, where the main result for the existence of Eq. (1) is proved.
2 Main result
Consider the Cauchy problem for Eq. (1)
which is equivalent to
In fact, as proved in , problem (2) satisfies the following conservation law:
Now we introduce the definition of a weak solution to Cauchy problem (2) or (3).
The main result of this paper is stated as follows.
3 Viscous approximations
and setting the mollifier with and , we know that for any , (see Lai and Wu ). In fact, choosing the mollifier properly, we have
Now we start our analysis by establishing the following well-posedness result for problem (9).
Proof For any and , we have . From Theorem 2.3 in , we conclude that problem (9) has a unique solution for an arbitrary .
We know that the first equation in system (9) is equivalent to the form
from which we derive that
which completes the proof. □
From Lemma 3.1 and (8), we have
Due to strong similarities with the proof of Lemma 5.1 presented in Coclite et al., we do not prove Lemma 3.3 here.
Lemma 3.4Assume thatis the unique solution of (9). For an arbitrary, there exists a positive constantCdepending only onand the coefficients of Eq. (1) such that the following one-sidednorm estimate on the first-order spatial derivative holds:
The proofs of Lemmas 3.5 and 3.6 are similar to those of Lemmas 5.2 and 5.3 in . Here we omit their proofs.
Proof (28) and (29) are a direct consequence of Lemmas 3.1 and 3.2. Inequality (30) is valid because of the weak convergence in (29). Finally, (31) is a consequence of the definition of , Lemma 3.5 and (28). □
Remark 3.9 From (28) and (29), we know that
The next lemma contains a generalized formulation of (38).
Now, we will prove the strong convergence result, i.e.,
We do not provide the proofs of Lemmas 4.4 and 4.5 since they are similar to those of Lemmas 6.4 and 6.5 in Coclite et al..
Applying Lemmas 4.4 and 4.5 gives rise to
By Remark 3.9 and Lemma 4.3, one has
Using (51) derives
Then, from (50) and (55), we have
which completes the proof. □
Proof of the main result Using (8), (10) and Lemma 3.5, we know that the conditions (i) and (ii) in Definition 1 are satisfied. We have to verify (iii). Due to Lemma 4.2 and Lemma 4.6, we have
From Lemma 3.5, (27) and (58), we know that u is a distributional solution to problem (3). In addition, inequalities (5) and (6) are deduced from Lemmas 3.2 and 3.4. The proof of Theorem 1 is completed. □
The authors declare that they have no competing interests.
The article is a joint work of three authors who contributed equally to the final version of the paper. All authors read and approved the final manuscript.
Thanks are given to referees whose comments and suggestions were very helpful for revising our paper. This work is supported by both the Fundamental Research Funds for the Central Universities (JBK120504) and the Applied and Basic Project of Sichuan Province (2012JY0020).
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