1. Introduction
We consider the following variation of the two-component rod system:
()
If we introduce a momentum
y =
u −
uxx, the previous system possesses the following form:
()
where
()
with
u(
x,
t) and
ρ(
x,
t) depending on a space variable
x ∈
𝕊 =
ℝ/
ℤ and a time variable
t. It is obvious that system (
1) for
π(
ρ) = 0 reduces to the rod equation studied in [
1–
12]. Particularly, for
π(
ρ) = 0 and
σ = 1, (
1) becomes the celebrated Camassa-Holm equation which was investigated by many authors [
13–
21]. It reduces to the two-component rod system for
μ(
ρ) = 0 studied in [
22]. Moreover, system (
1) reduces to the two-component Camassa-Holm (CH2) equation for
μ(
ρ) = 0 and
σ = 1 which was investigated in [
23–
30]. The two-component rod system which includes both velocity and density variables in the dynamics possesses the following form:
()
We note that the geometric structure of (
1) is different from (
4) with the incorporation of the term
μ(
ρ); this restricts our discussion only on the unit circle. Furthermore, the discussion of this work shows that system (
1) possesses wave breaking phenomenon which is described with different blow-up criteria, while system (
4) admits not only breaking wave solutions but also global in time solutions. However, we do not know whether the global solutions of (
1) exist or not for the time being. It is the structure of (
1) that breaks some properties which previously holds for (
4). It is also known that for (
4)
()
which follows that ∫
𝕊 ρdx is an invariant with respect to time. Particularly, if
ρ has zero mean initially, then the solution
ρ to (
4) will preserve zero mean for all time. This motivates us to introduce the term
π(
ρ) since it always has zero mean. On the other hand, we consider the system (
1) in the spaces
Hs ×
Hs−1/
ℝ for
s > 5/2 on the circle, where
Hs =
Hs(
𝕊) denotes the
L2-Sobolev space of regularity, and denote by
Hs−1/
ℝ the space
Hs−1 with two functions being identified if they differ by a constant. The basic idea of this variation is the decomposition of
ρ ∈
Hs−1 into
π(
ρ) and
μ(
ρ), where
μ(
ρ) ∈
ℝ is independent on variable
x,
π(
ρ) belongs to
, a subspace of
Hs−1 containing all zero mean functions. The main purpose of our work is to investigate formation of singularities of solutions to (
1) where the conservation laws play crucial roles. The idea is motivated by Guo′s recent works [
28,
31].
The rest of this paper is organized as follows. In Section 2, we recall the local well-posedness theorem and show some auxiliary results which will be used in the sequel. In Section 3, the detailed blow-up criteria are established via various initial conditions and as a byproduct, the blow-up rate is presented.
2. Preliminaries
In this section, we would like to list some useful results for later use. We now provide the framework in which we shall reformulate (
1). Let
()
Then
for all
f ∈
L2(
𝕊) and
G*
y =
u, where * is the spatial convolution. With this in hand, we rewrite (
1) as follows:
()
Firstly, we recall the elementary result on the local well-posedness theorem for system (
7) which was shown in [
32] from the geometric formulation.
Theorem 1. Let s > 5/2. There is an open neighborhood U containing (0, π(0)) ∈ Hs × Hs−1/ℝ such that for any (u0, π(ρ0)) ∈ U there exists a maximal T > 0 and a unique solution (u, π(ρ)) to the initial value problem for system (7) with
()
(
u,
π(
ρ))(0) = (
u0,
π(
ρ0)). Moreover, the solution depends continuously on the initial data, that is, the mapping
()
is continuous.
We now introduce two characteristics for (
7)
()
()
where
u denotes the first component of the solution to (
7) with certain initial data and
T is the lifespan of the solution, then
qi (
i = 1,2) is a diffeomorphism of the line. These two characteristics are both increasing diffeomorphisms of
ℝ. A direct calculation shows
()
Thus, we have
()
()
This is usually called the particle trajectory method, and it is important in the discussion of blow-up phenomena. We are now in a position to state the following.
Lemma 2. Let X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s > 5/2, and let T be the maximal existence time of the solution X = (u, π(ρ)) to (7) with the initial data X0. Then for all (x, t) ∈ 𝕊 × [0, T), we have
()
Proof. Differentiating the left-hand side of (15) with respect to t, we obtain
()
where we have used
q1 defined in (
10) and the second equation of system (
7). Then
π(
ρ(
q1,
t))
q1,x(
x,
t) is independent on time
t. Now we choose
t = 0, due to (
13) we know
q1,x(
x, 0) = 1. Therefore, this lemma is easily proved.
The application of (15) leads to the following result.
Theorem 3. Let X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s > 5/2, and let T be the maximal existence time of the solution X = (u, π(ρ)) to (7) with the initial data X0. If there exists M > 0 such that
()
then
Hs ×
Hs−1-norm of
X does not blow up on [0,
T).
Proof. Applying the operator Λs where to the first equation in (7), and multiplying by Λsu, then integrating over 𝕊, we obtain
()
where
()
From the proof in [
25], we get
()
For the third term on the right-hand side of (
13), we estimate it as follows:
()
Combining the previous inequalities, we can get
()
where the constant
c may be different from instance to instance. We do similar estimates for the second component by applying the operator Λ
s−1 to the second equation in (
7), and multiplying by Λ
s−1π(
ρ), then integrating over
𝕊 to obtain
()
Similarly, we have
()
Therefore, it follows that
()
By (
22) and (
25) we have
()
It is easy to obtain by Gronwall′s inequality, Lemma
2, and (
13) that
()
Then the
Hs ×
Hs−1-norm of
X does not blow up on [0,
T).
Next we present the precise blow-up scenario for sufficiently regular solutions to (7).
Theorem 4. Let X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s > 5/2, and let T be the maximal existence time of the solution X = (u, π(ρ)) to (7) with the initial data X0. Then the solution X blows up in finite time if and only if
()
Proof. Multiplying the first equation in (2) by y and integrating by parts, we get
()
It then follows that
()
Differentiating the first equation in (
2) with respect to
x, multiplying by
yx, and integrating by parts yield
()
Combining (
30) and (
31) together, we obtain
()
Similar arguments made on the second equation in (
2) yield
()
It follows by combining (
32) and (
33) that
()
Assume that there exist
M1 > 0 and
M2 > 0, such that
()
By Lemma
2, we have
()
It then follows that
()
It is easy to obtain by Gronwall′s inequality that
()
Sobolev imbedding, (
38), and Theorem
3 ensure that the solution
X does not blow up in finite time.
On the other hand, due to the Sobolev imbedding theorem, we observe that ux(x, t)→−∞ or will lead to blow-up of solutions.
After the local well-posedness of strong solutions (see Theorem 1) is established, a natural question is whether this local solution can exist globally. If the solution only exists in finite time, what induces the blow-up phenomenon? On the other hand, finding sufficient conditions to guarantee the finite time singularities or global existence is very interesting, especially for sufficient conditions added on certain initial data. The following results will give some positive answers.
3. Blow-Up
In this section, we pay more attention to the formation of singularities for strong solutions to our system. It will show that wave breaking is one way that singularities arise in smooth solutions. We start this section with the following lemmas.
Lemma 5. Let X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s ≥ 5/2, and let T be the maximal existence time of the solution X = (u, π(ρ)) to (7) with the initial data X0. Then we obtain the following conservation laws:
()
Proof. The proofs are direct consequences of the energy method; similar ones are given in [28] for the two-component Camassa-Holm equations; we refer the readers to [28] for the details.
Remark 6. The conservation of E1 guarantees the uniform bound of u(x, t), then Theorem 4 is also interpreted as wave breaking.
Lemma 7 (see [10].)For all f ∈ H1(𝕊), the following inequality holds:
()
with
()
Moreover,
C0 is the optimal constant obtained by the function
()
Lemma 8 (see [33].)For all f ∈ H1(𝕊), the following inequality holds:
()
where
()
Moreover,
C1 is the minimum value, so in this sense,
C1 is the optimal constant which is obtained by the associated Green′s function
()
Lemma 9 (see [33].)For any function f ∈ H2(𝕊), the following inequality holds:
()
Lemma 10 (see [34].)Assume f(x) ∈ Hs(𝕊), s > 2. If ∫𝕊 f(x)dx = 0, then
()
It is now to state our result.
Theorem 11. Let X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s ≥ 5/2, and let T be the maximal existence time of solution X = (u, π(ρ)) to (7) with the initial data X0. If the following inequality holds:
()
where
E1(0) is the initial value of
E1 in Lemma
5, and constant
κ is determined later. Then the corresponding solution
X blows up in finite time.
Proof. Differentiating the first equation in system (7) with respect to x, we obtain
()
Applying the relation
to (
49) gives
()
If 0 <
σ < 1,
multiplying (
50) with
and integrating by parts subsequently, we obtain
()
where we have used the following identity:
()
Since 3 − 3
σ > 0, and
()
where
C0 ≈ 0.869 < 1.
In the following, we estimate the three terms on the right-hand side of (51) one by one. The Cauchy-Schwarz inequality implies that
()
hence
()
Using Lemma
8 and the invariant property of
E1, we have
()
Note the fact that
()
This implies that for any
t ∈ [0,
T),
π(
ρ) has at least a zero point
ηt, that is,
π(
ρ(
ηt,
t)) = 0. Therefore, we have
()
Suppose that the solution does not blow-up in finite time, it follows that there exists a constant
M* > 0 such that
ux(
x,
t)>−
M* and
is bounded by
M*; thus
()
Then
()
By (
55)–(
60), we have
()
For convenience, we denote
()
where we noticed the fact that
()
That is,
()
Note that if the initial quantity satisfies
()
the standard argument on the Riccati type inequality and the initial hypothesis ensure that there exists a finite time
T such that
()
Since
()
This implies that
()
Then it contradicts the assumption
ux(
x,
t)>−
M*. By Theorem
4, we know that the solution must blow up in finite time.
If 1 ≤ σ < 3, we get
()
We also denote that
()
It follows that
()
where we note 3(3 −
σ)(1 −
C0)/2 > 0. Similar to case (1), it is easy to see that under the corresponding condition (2) of our theorem and the previous arguments, blow-up phenomenon occurs. We complete the proof.
As we know, the key issue for partial differential equations lies in the estimates. In the following results, we apply different strategies to derive suitable bounds for solution, then blow-up phenomenon occurs while some special initial values are involved. Precisely, we show the following.
Theorem 12. Suppose that X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s ≥ 5/2, u0 does not vanish identically. If ∫𝕊 u0(x)dx = 0, and one of the following conditions is satisfied
()
for some constant
C2 and function
φ(
t) is defined as previously, then the corresponding solution to initial data
X0 of (
7) blows up in finite time.
Proof. Differentiating both sides of the first equation of (7) with respect to variable x, we obtain
()
Applying the relation
to (
73) gives
()
Multiplying (
74) with
and integrating by parts subsequently, we obtain
()
where we have used the following identity:
()
On the other hand, we know that ∫
𝕊 u(
x,
t)
dx = 0 in view of the hypothesis, and the following inequality holds
()
where
()
Using Lemma
10 and (
77), we obtain
()
In view of (
60), we obtain that
()
If 3sinh (1/2)/(6 + sinh (1/2)) <
σ < 3, it is easy to know that
()
If there exists a constant
C2 such that the initial energy
E1(0) >
C2, then there is some
δ > 0 such that
()
On the other hand, we have by Lemma
10
()
Therefore, the previous inequality yields
()
In view of Hölder′s inequality, there holds
()
For simplicity of notations, we denote by
φ(
t) and
μ the following quantities:
()
respectively. Therefore we have
()
First, we can easily get
, and it is not difficult to find that there exists a
t0 ≥ 0 such that
φ(
t0) < 0. Then for
t >
t0, we get
()
Solving this inequality yields
()
which approaches −
∞ as
t arrives at
T0 = −(6/
σ)
φ−1/3(
t0) +
t0; that is, there exists a time
T ≤ −(6/
σ)
φ−1/3(
t0) +
t0 such that
()
Therefore, it follows that
()
Then it contradicts the assumption
ux(
x,
t)>−
M*. By blow-up scenario, we know that the solution must blow up in finite time.
If 0 < σ ≤ 3sinh (1/2)/(6 + sinh (1/2)), then we have
()
where
()
We also use
φ(
t) as previously to get
()
Similar arguments to (
64) in Theorem
11 and condition (2) guarantee the finite time blow-up of solution to (
7).
The zero mean of u(x, t) in the previous theorem can be substituted by E2(0) = 0; blow-up still occurs with the aid of different estimate from (83).
Theorem 13. Assume that X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s ≥ 5/2, u0 does not vanish identically, , if one of the following conditions is satisfied:
()
for some constant
C3, then the corresponding solution to (
7) blows up in finite time.
Proof. By assumption and the invariance property of E2, we have
()
Therefore,
u(
x,
t) must change sign, so there exists at least one zero point on
𝕊. Then for each
t ∈ [0,
T), suppose that there is a
ξt ∈ [0,1] such that
u(
ξt,
t) = 0, for
x ∈
𝕊 we have
()
Thus, the previous relation and an integration by parts yield
()
Doing a similar estimate on [
ξt + 1/2,
ξt + 1], we obtain
()
In view of (
97), we also have
()
Let
()
we obtain
()
where we use the fact 1/2sinh (1/2) ≤
G(
x) ≤ cosh (1/2)/2sinh (1/2), then
()
For
σ ∈ (3sinh (1/2)/(2 + sinh (1/2)), 3), we have
()
If there is a constant
C3 such that the initial energy
E1(0) >
C3, there exists some
δ > 0 such that
()
On the other hand, we have
. Therefore, the previous inequality yields
()
The remaining part is very close to Theorem
12, so we omit it. So the solution must blow up in finite time.
For σ ∈ (0,3sinh (1/2)/(2 + sinh (1/2))], then
()
where 3(3 −
σ)/8 − 3
σ/4sinh (1/2) ≥ 0. Note that if the initial quantity satisfies
()
then condition (2) can conclude that the solution to (
7) goes to −
∞ in finite time. This completes the proof.
Theorem 14. Suppose that X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s ≥ 5/2, X = (u, π(ρ)) is the solution to system (7) with the initial data X0. If there is some point x0 ∈ 𝕊 such that one of the following conditions is satisfied:
()
where the constants
C0 and
C1 are given in Lemmas
7 and
8,
E1(0) is the initial value of
E1, then the solution
X must blow up in finite time.
Proof. Differentiating the first equation in system (7) with respect to x and noticing that , we have
()
When 0 <
σ < 1, this equation, in combination with (
11), yields
()
Note that
()
We can deduce that there exists at least one point
x0 such that
π(
ρ(
q2(
x0,
t),
t)) = 0 for
t ∈ [0,
T). Let us consider this problem at (
q2(
x0,
t),
t). For convenience, we denote
ux(
q2(
x0,
t),
t) =
m(
t). Then we have
()
Using the notation
, where 3 −
σ − 2
σC0 > 0, we have
()
In view of the initial condition, it is not difficult to obtain
()
with 0 <
δ < 1 determined by
δm2(0) =
ϱ. Then, by using the standard arguments for this type of inequality and our hypothesis, it is easy to conclude that the lifespan of the solution is finite; that is, blow-up phenomenon occurs.
When 1 ≤ σ < 3, then
()
where (3 −
σ)(1 −
C0) > 0. Similar to the arguments about (
113), under the condition (2), the corresponding solution
X blows up in finite time. We complete the proof.
Theorem 15. Suppose that X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s ≥ 5/2, X = (u, π(ρ)) is the solution to system (7) with the initial data X0. If there holds one of the following conditions for some point x0 ∈ 𝕊
()
where the constant
C0 is the best constant given by Lemma
7 and
()
Let
T be the maximal existence time of the corresponding solution to (
7) with the initial data
X0. Then
T is finite.
Proof. We easily know that ∫𝕊 u(x, t)dx is also an invariant with respect to time. The result follows by using Lemma 9 that
()
in (
113) instead of
.
In the following, as a byproduct, we examine the blow-up rate while the solution blows up in finite time.
Theorem 16. Assume that X0 = (u0, π(ρ0)) ∈ Hs × Hs−1/ℝ, s ≥ 5/2, X = (u, π(ρ)) is the corresponding solution with the initial data X0. If there holds the condition of Theorem 14, then we have the following description:
()
where
σ > 0 and
m(
t) is defined in Theorem
14.
Proof. The conclusion follows from the theory of ordinary differential equations to inequality (113). Indeed, when 0 < σ < 1, by (113) we have
()
In view of Lemma
8 and the conservation of
E1, there holds for all
t ∈ [0,
T) that
()
It follows that
()
Since lim
t→Tm(
t) = −
∞ by Theorem
14, it implies that for any
ε ∈ (0,
σ/2) there exists a
t0 such that
m2(
t) >
σϱ/2
ε for all
t ∈ [
t0,
T). Therefore,
()
that is,
()
Direct integration from
t to
T gives
()
and the arbitrariness of
ε leads to our result.
When 1 ≤ σ < 3, by (116) we obtain
()
where
ω = ((3 −
σ)/
σ)(1 −
C0)
C1E1(0). Similar to the arguments for 0 <
σ < 1, we can get the same result.