ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu ftp ejde.math.txstate.edu

WAVE-BREAKING PHENOMENA AND GLOBAL SOLUTIONS FOR PERIODIC TWO-COMPONENT

DULLIN-GOTTWALD-HOLM SYSTEMS

MIN ZHU, JUNXIANG XU

Abstract. In this article we study the initial-value problem for the periodic two-componentb-family system, including a special case, whenb= 2, which is referred to as the two-component Dullin-Gottwald-Holm (DGH) system.

We first show that the two-componentb-family system can be derived from the theory of shallow-water waves moving over a linear shear flow. Then we establish several results of blow-up solutions corresponding to only wave breaking with certain initial profiles for the periodic two-component DGH system. Moreover, we determine the exact blow-up rate and lower bound of the lifespan for the system. Finally, we give a sufficient condition for the existence of the strong global solution to the periodic two-component DGH system.

1. Introduction

In recent years, Degasperis, Holm and Hone [22] (see also [33]) studied the fol- lowing nonlinearb-family equation (up to a rescaling, shift and Galilean’s transfor- mation),

mt−Aux+umx+buxm+γuxxx= 0, x∈R, t >0, (1.1)
wherem=u−α^{2}uxx.One can rewrite equation (1.1) in terms ofu(x, t) as follows:

ut−α^{2}uxxt−Aux+(b+1)uux+γuxxx=α^{2}(buxuxx+uuxxx), x∈R, t >0. (1.2)
This equation can be regarded as a model of water waves by using asymptotic
expansions directly in the Hamiltonian for Euler’s equation in the shallow water
regime [20, 33], where u(t, x) stands for the horizontal velocity of the fluid, m is
the momentum density, and A is a nonnegative parameter related to the critical
shallow water speed. The real dimensionless constantb is a parameter which pro-
vides the competition, or balance, in fluid convection between nonlinear steepening
and amplification due to stretching, it is also the number of covariant dimensions
associated with the momentum densitym.

It is believed that the Korteweg-de Vries (KdV) equation (α= 0 and b = 2), the Camassa-Holm (CH) equation (b = 2) [4, 26] (when b = 2 and γ 6= 0, it is also referred to as the Dullin-Gottwald-Holm (DGH) equation [4, 20]), and the

2000Mathematics Subject Classification. 35B30, 35G25.

Key words and phrases. Two-component Dullin-Gottwald-Holm system;

periodic two-componentb-family system; blow-up; wave-breaking; global solution.

c

2013 Texas State University - San Marcos.

Submitted November 14, 2012. Published February 8, 2013.

1

Degasperis-Procesi (DP) equation (b= 3) [23] are the only three integrable equa-
tions in theb-family equation (1.2) [20, 21, 22, 23, 33, 34]. WhenA=γ= 0, (1.2)
admits not only the peakon solutions for anybof the formu(t, x) =ce^{−|x−ct|},c∈R,
but also multipeakon solutions [1, 22, 33] (see also [6] for the case of existence of
infinite many peakons) defined by

u(x, t) =

N

X

j=1

pj(t)e^{−|x−q}^{j}^{(t)|},

where the canonical positionsqj and momenta pj (with j = 1, . . . , N) satisfy the following system of ordinary differential equations with discontinuous right-hand side.

p^{0}_{j}= (b−1)

N

X

k=1

p_{j}p_{k}sgn(q_{j}−q_{k})e^{−|q}^{j}^{−q}^{k}^{|}
and

q_{j}^{0} =

N

X

k=1

p_{k}e^{−|q}^{j}^{−q}^{k}^{|}.

Ifα= 0 andb= 2, equation (1.2) becomes the well-known KdV equation which describes the unidirectional propagation of waves at the free surface of shallow water under the influence of gravity. Its solitary waves are solitons. The Cauchy problem of the KdV equation has been the subject of a number of studies, and a satisfactory local or global existence theory is now in hand [45]. It is observed that the KdV equation does not accommodate wave breaking (by wave breaking we understand that the wave profile remains bounded while its slope becomes unbounded in finite time [47]).

When b = 2 and γ = 0, equation (1.2) recovers the standard CH equation, modeling the unidirectional propagation of shallow water waves over a flat bottom [4, 13, 26]. The CH equation is also a model for the propagation of axially symmetric waves in the hyperelastic rods [19]. Its solitary waves are smooth if A > 0 and peaked in the limiting caseA= 0 [4, 5, 6]. Recently, it was claimed in [38] that the CH equation might be relevant to the modeling of tsunami.

Ifb= 3 andA=γ= 0 in equation (1.2), then it recovers the DP equation. The DP equation can be also regarded as a model for nonlinear shallow water dynamics and its asymptotic accuracy is the same as the CH equation [13]. The formal integrability of the DP equation was obtained in [22] by constructing a Lax pair.

It has a bi-Hamiltonian structure. The DP equation has not only peaked solitons and periodic peaked solitons, but also shock peakons [43] and the periodic shock waves [25].

The CH and DP equations have global strong solutions and also blow-up solu-
tions in finite time, for instance, see [7, 9, 10, 14, 25, 40, 41, 42] and references
therein, with a different class of initial profiles in the Sobolev spacesH^{s}(R), s >

3/2. It is shown in [2] and [3] that solutions of the CH equation can be uniquely continued after breaking as either global conservative or global dissipative weak solutions. The advantage of the CH and DP equations in comparison with the KdV equation lies in the fact that the CH and DP equations have peaked soli- tons and models wave breaking. Wave breaking is one of the most intriguing long-standing problems of water wave theory [47]. The peaked solitons are the presence of solutions in the form of peaked solitary waves or ”peakons” [4, 5, 6, 23]

u(t, x) = ce^{−|x−ct|}, c6= 0, which are smooth except at the crests, where they are
continuous, but have a jump discontinuity in the first derivative. The peakons
replicate a feature that is characteristic for the waves of great height-waves of the
largest amplitude that are exact solutions of the governing equations for water
waves [8, 46, 11]. These peakons are shown to be stable [15, 16, 39].

The interest in the b-family equation inspired the search for various general- izations of this equation. The following two-component integrable Camassa-Holm system was first derived in [44] and can be viewed as a model in the context of shallow water theory [12, 35],

m_{t}−Au_{x}+um_{x}+ 2u_{x}m+ρρ_{x}= 0,
m=u−uxx,

ρt+ (uρ)x= 0,

(1.3)

whereρ(t, x) is related to the free surface elevation from equilibrium(or scalar den- sity), and the parameterAcharacterizes a linear underlying shear flow. Obviously, if ρ = 0, then (1.3) becomes the CH equation. Many recent works are devoted in studying system (1.3) (see, for instance, [12, 24, 27, 29, 30, 31, 32, 35, 49] and references therein).

In the presence of a linear shear flow and nonzero vorticity, we will follow Ivanov’s approach [35] to derive the following two-componentb-family system with anyb6=

−1.

m_{t}−Au_{x}+um_{x}+bu_{x}m+γu_{xxx}+ρρ_{x}= 0,
m=u−uxx,

ρt+ (uρ)x= 0.

(1.4) Note when ρ= 0, we recover theb-family equation (1.1). In terms ofuand ρ, we obtain the equivalent form of system (1.4); that is,

u_{t}−u_{txx}−Au_{x}+ (b+ 1)uu_{x}−bu_{x}u_{xx}−uu_{xxx}+γu_{xxx}+ρρ_{x}= 0,

ρt+ (uρ)x= 0, (1.5)

with the boundary assumptionsu→0 andρ→1 as|x| → ∞.

Note that when b= 2, equation(1.5) is the two-component Camassa-Holm sys- tem, which has the bi-Hamiltonian structure and complete integrability via the inverse scattering transform method. It can be written as compatibility conditions of two linear systems (Lax pair) with a spectral parameterξ, that is

Ψxx=

−ξ^{2}ρ^{2}+ξ
m−A

2 +γ 2

+1 4

Ψ, Ψt= 1

2ξ−u+γ Ψx+1

2uxΨ.

Moreover, this system has the following two Hamiltonians E(u, ρ) =1

2 Z

u^{2}+u^{2}_{x}+ (ρ−1)^{2}
dx
and

F(u, ρ) =1 2

Z

u^{3}+uu^{2}_{x}−Au^{2}−γu^{2}_{x}+ 2u(ρ−1) +u(ρ−1)^{2}
dx.

The goal of this article is to study the initial-value problem for the periodic
two-componentb-family system, including a special case, b= 2, which is the two-
component DGH system. We first derive the two-componentb-family system from
the shallow-water wave theory. Then we establish several results of blow-up so-
lutions corresponding to only wave breaking with certain initial profiles for the
periodic two-component DGH system. The difficulty to deal with blow-up solu-
tions is that there is no uniform characteristics for this system. In this case, we
make use of the different diffeomorphism of the trajectoryq_{2}defined in (4.4), which
captures the maximum/minimum ofux. Therefore the transport equation forρcan
coincide with the equation foru.

The rest of this paper is organized as follows. In Section 2, we follow the modeling approach in [35] to derive the two-component b-family system. Then applying Kato’s semigroup theory, we establish the result of local well-posedness for the two componentb-family system in Section 3. In Section 4, we analyze the wave-breaking phenomenon of the periodic two-component DGH system and give the precise blow- up scenarios and several wave-breaking data. In addition, we determine the blow- up rate and low bound of the lifespan. In the last section, we provide a sufficient condition for the existence of global solution.

Notation. Throughout this paper, we identity periodic functions with function
spaces over the unit circleSin R^{2}, i.e. S=R/Z.

2. Derivation of the model

Following Ivanov’s approach in [35] , we consider the motion of an inviscid in- compressible fluid with a constant density%governed by the Euler equations

~vt+ (~v· ∇)~v=−1

%∇P+~g,

∇ ·~v= 0,

where ~v(t, x, y, z) is the velocity of the fluid, P(t, x, y, z) is the pressure and~g = (0,0,−g) is the gravity acceleration.

Using the shallow water approximation and non-dimensionalization, the above equations can be written as

ut+ε(uux+wuz) =−px,
δ^{2}(wt+ε(uwx+wwz)) =−pz,

u_{x}+w_{z}= 0,

w=ηt+εuηx, p=η onz= 1 +εη, w= 0 onz= 0,

where~v = (u,0, w) andp(x, z, t) is the pressure variable measuring the deviation from the hydrostatic pressure distribution andη(t, x) is the deviation from the mean level z=hof the water surface. ε=a/hand δ=h/λare the two dimensionless parameters withabeing the typical amplitude of the wave andλbeing the typical wavelength of the wave.

In the presence of an underlying shear flow, the horizontal velocity of the flow becomes u+ ˜U(z). We take the simplest case ˜U(z) = Az in which A > 0 is a constant. Notice that the Burns condition gives the shallow-water limit of the dispersion relation for the waves with vorticity , hence determines the speed of

propagation of the linear waves. From Burns condition [17, 28] one has the following expression for the speedcof the traveling waves in linear approximation,

c= 1 2

A±p

4 +A^{2}

. (2.1)

In the case of the constant vorticity ω =A, we obtain the following equations
foru0 andη by ignoring the terms ofO(ε^{2}, δ^{4}, εδ^{2})

u0−1

2δ^{2}u0,xx

t

+εu0u0,x+ηx−A

3δ^{2}u0,xxx= 0, (2.2)
ηt+Aηx+

(1 +εη)u0+A
2εη^{2}

x−1

6δ^{2}u0,xxx= 0, (2.3)
whereu_{0}is the leading order approximation foru(see the details in [35]). Let both
of the parametersandδ go to 0. Then by (2.2) and (2.3), we have the system of
linear equations

u0,t+ηx= 0,
η_{t}+Aη_{x}+u_{0,x}= 0.

This in turn implies thatηtt+Aηtx−ηxx= 0. Introducing a new variable
ρ= 1 +εαη+ε^{2}βη^{2}+εδ^{2}µu_{0,xx},

for some constantsα, β andµsatisfying µ

α= 1

6(c−A), α= 1 +Ac

2 +β α, then equations (2.2) and (2.3) become

mt+Amx−Au0,x− 1

6c^{2}(c−A)δ^{2}u0,xxx

+ε

1−α^{2}+ 2β
α c^{2}

u0u0,x+ 1

2εα(ρ^{2})x= 0,
ρt+Aρx+αε(ρu0)x= 0,

(2.4)

wherem=u_{0}−^{1}_{2}δ^{2}u_{0,xx}. Sinceb6=−1 and

(b+ 1)u_{0}u_{0,x}=bmu_{0,x}+u_{0}m_{x}+O(δ^{2}),
equation (2.4) can be reformulated at the order ofO(ε, δ^{2}) as

m_{t}+Am_{x}−Au_{0,x}− 1

6c^{2}(c−A)δ^{2}u_{0,xxx}

+ ε

b+ 1

1−α^{2}+ 2β
α c^{2}

(bmu0,x+u0mx) + 1

2εα(ρ^{2})x= 0.

Using the scalingu0→ _{αε}^{1}u0, x→δxand t→δt, then (2.4) becomes
mt+Amx−Au0,x− 1

6c^{2}(c−A)u0,xxx+
1

(b+ 1)α

1−α^{2}+ 2β
α c^{2}

(bmu0,x+u0mx) +1

2(ρ^{2})x= 0,
m=u_{0}−u_{0,xx},

ρt+Aρx+ (ρu0)x= 0.

Now if we choose

1 (b+ 1)α

1−α^{2}+ 2β
α c^{2}

= 1
and denoteγ=−_{6c}2(c−A)^{1} , then we arrive at

mt+Amx−Au0,x+bmu0,x+u0mx+γu0,xxx+ρρx= 0,
m=u_{0}−u_{0,xx},

ρt+Aρx+ (ρu0)x= 0.

(2.5) Thus the constantsα, β, µandcsatisfy

α=c^{2}(c^{2}+ 1) + 1

3c^{2}+b+ 1 , β=α^{2}−α
1 + Ac

2

,

µ= α

6(c−A), c^{2}−Ac−1 = 0.

With a further Galilean transformation x → x−ct, t → t, we can drop the
termsAρ_{x} andAm_{x} in (2.5) and obtain the two-componentb-family system (1.4)
or (1.5).

3. Local well-posedness

In this section, we will apply Kato’s semigroup theory to establish the local well-posedness for the following periodic initial-value problem to (1.5).

u_{t}+ (u−γ)u_{x}=−∂x(1−∂_{x}^{2})^{−1}b

2u^{2}+3−b

2 u^{2}_{x}+ (γ−A)u+1
2ρ^{2}

, t≥0, x∈R,

ρ_{t}+ (uρ)_{x}= 0, t≥0, x∈R,
u(0, x) =u0(x), x∈R,
ρ(0, x) =ρ0(x), x∈R,
u(t, x+ 1) =u(t, x), t≥0, x∈R,
ρ(t, x+ 1) =ρ(t, x), t≥0, x∈R.

(3.1)

For convenience, we present here Kato’s theorem in a form suitable for our purpose. Consider the abstract quasilinear evolution equation

dv

dt +A(v)v=f(v) t≥0,
v(0) =v_{0}.

(3.2)
LetX andY be two Hilbert spaces such that Y is continuously and densely em-
bedded inX and letQ:Y →X be a topological isomorphism. LetL(Y, X) denote
the space of all bounded linear operators fromY to X, particularly, it is denoted
byL(X) ifX =Y. The linear operatorA belongs toG(X,1, β) whereβ is a real
number, if −A generates a C0-semigroup such that ke^{−sA}k_{L(X)} ≤e^{βs}. We make
the following assumptions, whereµi(1 = 1,2,3,4) are constants depending only on
max{kykY, kzkY}:

(i)A(y)∈L(Y, X) fory∈Y with k A(y)−A(z)

wkX ≤µ_{1}ky−zkXkwkY, y, z, w∈Y

andA(y)∈G(X,1, β) (i.e.,A(y) is quasi-m-accretive), uniformly on bounded sets inY.

(ii) QA(y)Q^{−1} = A(y) +B(y), where B(y) ∈ L(X) is bounded, uniformly on
bounded sets inY. Moreover,

k B(y)−B(z)

wkX ≤µ2ky−zkYkwkX, y, z∈Y, w∈X.

(iii) f : Y →Y extends to a map fromX into X, is bounded on bounded sets in Y, and satisfies

kf(y)−f(z)kY ≤µ3ky−zkY, y, z∈Y and

kf(y)−f(z)kX≤µ4ky−zkX, y, z∈Y.

Lemma 3.1([36]). Assume conditions(i), (ii) (iii)hold. Given v_{0}∈Y, there is a
maximalT >0depending only onkv_{0}k_{Y} and a unique solutionv to(3.2)such that

v=v(·, v0)∈C [0, T);Y

∩C^{1} [0, T);X
.

Moreover, the map v_{0} 7→ v(·, v0) is a continuous map from Y to C [0, T);Y

∩
C^{1} [0, T);X

.

We now provide the framework in which we shall reformulate problem (3.1).

Theorem 3.2. Given an initial data (u0, ρ0) ∈ H^{s}(S)×H^{s−1}(S), s ≥ 2, there
exists a maximalT =T k(u0, ρ0)k_{H}s(S)×H^{s−1}(S)

>0 and a unique solution
(u, ρ)∈C [0, T);H^{s}(S)×H^{s−1}(S)

∩C^{1} [0, T);H^{s−1}(S)×H^{s−2}(S)
of system (3.1). Moreover, the solution (u, ρ) depends continuously on the initial
value (u_{0}, ρ_{0})and the maximal time of existenceT >0 is independent of s.

The remaining of this section is devoted to the proof of Theorem 3.2. Let U =

u ρ

, A(U) =

(u−γ)∂x 0

0 u∂x

(3.3) f(U) =

−∂_{x}(1−∂^{2}_{x})^{−1} _{2}^{b}u^{2}+^{3−b}_{2} u^{2}_{x}+ (γ−A)u+^{1}_{2}ρ^{2}

−u_{x}ρ

(3.4)
Y =H^{s}×H^{s−1},X=H^{s−1}×H^{s−2}, Λ = (1−∂_{x}^{2})^{1/2} and

Q= Λ 0

0 Λ

.

Obviously,Qis an isomorphism ofH^{s}×H^{s−1} ontoH^{s−1}×H^{s−2}. Thus, to derive
Theorem 3.2, we only need to check that A(U) and f(U) satisfy the conditions
(i)-(iii), and this can be formulated through several lemmas.

The following lemmas from [36] and [37] are useful in our proofs.

Lemma 3.3 ([36]). Let r, tbe two real numbers such that −r < t≤r. Then
kf gkH^{t} ≤ckfkH^{r}kgkH^{t}, if r > 1

2 and

kf gk

H^{r+t−}^{1}2 ≤ckfk_{H}rkgk_{H}t, if r < 1
2,
wherec is a positive constant depending onr andt.

Lemma 3.4 ([37]). Let f ∈H^{r} for somer > ^{3}_{2}. Then

kΛ^{−¯}^{s}[Λ^{s+¯}^{¯} ^{t+1}, Mf]Λ^{−}^{¯}^{t}k_{L(L}2)≤ck∂xfk_{r−1}, |¯s|, |¯t| ≤r−1,

whereMf is the operator of multiplication byf andc is a constant depending only ons¯and¯t.

Lemma 3.5. With U ∈ H^{s}(S)×H^{s−1}(S)(s ≥ 2), the operator A(U) belongs to
G(H^{s−1}(S)×H^{s−2}(S),1, β).

Proof. Taking the H^{s−1}×H^{s−2} inner product withW =
w1

w2

on both sides of the equation

dW

dt +A(U)W = 0 gives

1 2

d

dtkWk^{2}_{H}s−1×H^{s−2}

=−hW, A(U)Wi(s−1)×(s−2)

=−D
w_{1}
w_{2}

,

(u−γ)∂_{x}w_{1}
u∂_{x}w_{2}

E

(s−1)×(s−2)

=−hw1,(u−γ)∂xw1is−1− hw2, u∂w2is−2

=−hΛ^{s−1}w1,Λ^{s−1} (u−γ)∂xw1

i − hΛ^{s−2}w2,Λ^{s−2} u∂xw2

i

=−hΛ^{s−1}w1,[Λ^{s−1}, u−γ]∂xw1i − hΛ^{s−1}w1,(u−γ)∂xΛ^{s−1}w1i

− hΛ^{s−2}w2,[Λ^{s−2}, u]∂xw2i − hΛ^{s−2}w2, u∂xΛ^{s−2}w2i

=−hΛ^{s−1}w1,[Λ^{s−1}, u−γ]∂xw1i −1

2hΛ^{s−1}w1, ux∂xΛ^{s−1}w1i

− hΛ^{s−2}w2,[Λ^{s−2}, u]∂xw2i −1

2hΛ^{s−2}w2, ∂xuΛ^{s−2}w2i

≤ kΛ^{s−1}w1k^{2}_{L}2k[Λ^{s−1}, u−γ]Λ^{2−s}kL(L^{2})+1

2kuxkL^{∞}kΛ^{s−1}w1kL^{2}

+kΛ^{s−2}w2k^{2}_{L}2k[Λ^{s−2}, u]Λ^{3−s}kL(L^{2})+1

2kuxkL^{∞}kΛ^{s−2}w2kL^{2}

≤c(kUkH^{s}+|γ|) kw1k^{2}_{H}s−1+kw2k^{2}_{H}s−2

=c(kUkH^{s}+|γ|)kWk^{2}_{H}s−1×H^{s−2},

where use has been made of Lemma 3.4 withr= 0,t¯=s−2 and ¯s= 0, ¯t=s−3,
respectively. By integrating both of sides in the above the estimate, it follows that
A(U)∈G H^{s−1}(S)×H^{s−2}(S),1, c(kukH^{s}+γ)

Lemma 3.6. The operatorA(U)defined by (3.3)belongs to tL(H^{s}×H^{s−1}, H^{s−1}×
H^{s−2}). Moreover

k(A(U)−A(V))Wk_{H}s−1×H^{s−2}≤µ1kU −Vk_{H}s×H^{s−1}kWk_{H}s×H^{s−1},

U, V, W ∈H^{s}×H^{s−1}. (3.5)

Proof. In view of (3.3), we have (A(U)−A(V))W =

(u−γ)∂x−(v1−γ)∂x 0 0 u∂x−v1∂x

w1

w2

=

(u−v_{1})∂_{x}w_{1}
(u−v_{1})∂_{x}w_{2}

.

SinceH^{s−1}(s≥2) is a Banach algebra, takingr=s−1,t=s−2 in Lemma 3.3,
we have

k(A(U)−A(V))Wk_{H}s−1×H^{s−2}

≤ k(u−v_{1})∂_{x}w_{1}kH^{s−1}+k(u−v_{1})∂_{x}w_{2}kH^{s−2}

≤cku−v1k_{H}^{s−1}(k∂xw1k_{H}^{s−1}+k∂xw2k_{H}^{s−2})

≤ckU −Vk_{H}s−1×H^{s−2}kWk_{H}s−1×H^{s−2}.

TakingV = 0 in (3.5), we deduce thatA(U)∈L H^{s}×H^{s−1}, H^{s−1}×H^{s−2}

.

Lemma 3.7 ([24]). Let B(U) =QA(U)Q^{−1}−A(U), for U ∈H^{s}×H^{s−1} (s≥2).

ThenB(U)∈L H^{s−1}×H^{s−2}
and

k(B(U)−B(V))Wk_{H}s−1×H^{s−2} ≤µ_{2}kU−Vk_{H}s×H^{s−1}kWk_{H}s−1×H^{s−2},
U, V ∈H^{s}×H^{s−1}, W ∈H^{s−1}×H^{s−2}.

Lemma 3.8 ([24]). Let U ∈H^{s}×H^{s−1} (s≥2). Then the operatorf(U)defined
by (3.4)is bounded on bounded sets in(H^{s−1}×H^{s−2}), and satisfies

(a) kf(U)−f(V)kH^{s}×H^{s−1}≤µ_{3}kU−VkH^{s}×H^{s−1},U, V ∈H^{s}×H^{s−1},
(b) kf(U)−f(V)kH^{s−1}×H^{s−2}≤µ_{4}kU −VkH^{s−1}×H^{s−2},U, V ∈H^{s}×H^{s−1}.
Proof of Theorem 3.2. The result follows from Lemmas 3.5–3.8.

4. Blow-up mechanism forb= 2

In this section, we investigate the problem of the wave-breaking phenomenon for the initial-value problem of the periodic two-component Dullin-Gottwald-Holm system which is a special case of (1.5) asb= 2.

4.1. Preliminaries. The periodic two-component Dullin-Gottwald-Holm system can be written as

u_{t}−u_{txx}−Au_{x}+γu_{xxx}+ 3uu_{x}−2u_{x}u_{xx}−uu_{xxx}+ρρ_{x}= 0, t >0, x∈R,
ρt+ (uρ)x= 0, t >0, x∈R,

u(0, x) =u0(x), x∈R,
ρ(0, x) =ρ_{0}(x), x∈R,
u(t, x+ 1) =u(t, x), t≥0, x∈R,
ρ(t, x+ 1) =ρ(t, x), t≥0, x∈R.

(4.1) Let G(x) = cosh(x−[x]−1/2)

2 sinh(1/2) , x ∈ S. Then (1−∂_{x}^{2})^{−1}f =G∗f for all f ∈ L^{2}(S),
u =G∗m and m = u−u_{xx}. Our system (4.1) can be written in the following

“transport” type

ut+ (u−γ)ux=−∂xG∗
u^{2}+1

2u^{2}_{x}+ (γ−A)u+1
2ρ^{2}

, t >0, x∈R, ρt+ (uρ)x= 0, t >0, x∈R,

u(0, x) =u_{0}(x), x∈R,
ρ(0, x) =ρ0(x), x∈R,
u(t, x+ 1) =u(t, x), t≥0, x∈R,
ρ(t, x+ 1) =ρ(t, x), t≥0, x∈R.

(4.2)

To study the wave-breaking problem, we now briefly give the needed results without proof to pursue our goal. We consider the following two associated Lagrangian scales of the system (4.1)

∂q_{1}

∂t =u(t, q_{1})−γ, 0< t < T,
q1(0, x) =x, x∈R,

(4.3)

and ∂q_{2}

∂t =u(t, q_{2}), 0< t < T,
q2(0, x) =x, x∈R,

(4.4)
whereu∈C^{1}([0, T), H^{s−1}(S)) is the first component of the solution (u, ρ) to (4.1).

Lemma 4.1 ([18, 12]). Let (u, ρ) be the solution of system (4.1)with initial data
(u_{0}, ρ_{0}) ∈ H^{s}(S)×H^{s−1}(S), s ≥ 2, and T the maximal time of existence. Then
(4.3)has a unique solution q_{1}∈C^{1}([0, T)×R,R)and (4.4)has a unique solution
q_{2}∈C^{1}([0, T)×R,R). These two solutions satisfyq_{i}(t, x+1) =q_{i}(t, x)+1,i= 1,2.

Moreover, the mapsq_{1}(t,·) andq_{2}(t,·)are increasing diffeomorphisms of Rwith
q_{1x}(t, x) = expZ t

0

u_{x}(τ, q_{1}(τ, x))dτ

>0, (t, x)∈[0, T)×R, q2x(t, x) = expZ t

0

ux(τ, q2(τ, x))dτ

>0, (t, x)∈[0, T)×R.

The above lemmas indicate that q1(t,·) : R → R and q2(t,·) : R → R are
diffeomorphisms of the line for eacht∈[0, T). Hence, theL^{∞}norm of any function
v(t,·)∈L^{∞}(S) is preserved under the family of diffeomorphismsq1(t,·) andq2(t,·)
witht∈[0, T); that is,

kv(t,·)kL^{∞}(S)=kv(t, q1(t,·))kL^{∞}(S)=kv(t, q2(t,·))kL^{∞}(S), t∈[0, T). (4.5)
Similarly, we have

x∈infS

v(t, x) = inf

x∈S

v(t, q1(t, x)) = inf

x∈S

v(t, q2(t, x)), t∈[0, T), (4.6) sup

x∈S

v(t, x) = sup

x∈S

v(t, q_{1}(t, x)) = sup

x∈S

v(t, q_{2}(t, x)), t∈[0, T). (4.7)
Lemma 4.2 ([24]). Let (u, ρ)be the solution of (4.1) with initial data (u_{0}, ρ_{0})∈
H^{s}(S)×H^{s−1}(S), s≥2, andT the maximal time of existence. Then we have

ρ(t, q2(t, x))q2x(t, x) =ρ0(x), (t, x)∈[0, T)×S. (4.8) Moreover if there existsx0∈Ssuch that ρ0(x0) = 0, thenρ(t, q2(t, x0)) = 0for all t∈[0, T).

Lemma 4.3([9]). LetT >0andv∈C^{1}([0, T);H^{2}(R)). Then for everyt∈[0, T),
there exists at least one pointξ(t)∈Rwith

m(t) := inf

x∈R

(vx(t, x)) =vx(t, ξ(t)).

The functionm(t)is absolutely continuous on(0, T)with dm(t)

dt =vtx(t, ξ(t)) a.e. on (0, T).

We may use the following lemma derived in [31] to establish the blow-up criterion of solution to (4.1).

Lemma 4.4. Let 0 < s < 1. Suppose that f_{0} ∈ H^{s}(S), g ∈ L^{1}([0, T];H^{s}(S)),
v, v_{x} ∈ L^{1}([0, T];L^{∞}(S)) and that f ∈ L^{∞}([0, T];H^{s}(S))TC([0, T];S^{0}(S)) solves
the one-dimensional linear transport equation

f_{t}+vf_{x}=g,
f(0, x) =f0(x).

Then f ∈ C([0, T];H^{s}(R)). More precisely, there exists a constant C depending
only onssuch that

kf(t)kH^{s} ≤ kf0kH^{s}+CZ t
0

kg(τ)kH^{s}dτ +
Z t

0

kf(τ)kH^{s}V^{0}(τ)dτ
.
Hence,

kf(t)kH^{s} ≤e^{CV}^{(t)}

kf0kH^{s}+C
Z t

0

kg(τ)kH^{s}dτ
,
whereV(t) =Rt

0(kv(τ)kL^{∞}+kvx(τ)kL^{∞})dτ.

The above lemma was proved using the Littlewood-Palay analysis for the trans- port equation and the Moser-type estimates. Using this result and performing the same argument as in [31], we can obtain the following blow-up criterion (up to a slight modification, the proof is omitted).

Theorem 4.5. Let(u, ρ)be the solution of system(4.1)with initial data(u_{0}, ρ_{0})∈
H^{s}(S)×H^{s−1}(S), s≥2, andT the maximal time of existence. Then

T <∞ ⇒ Z T

0

kux(τ)kL^{∞}(S)dτ =∞. (4.9)
We now give several useful conservation laws of strong solutions to (4.1).

Lemma 4.6. Let (u, ρ)be the solution of system (4.1) with initial data(u0, ρ0)∈
H^{s}(S)×H^{s−1}(S), s≥ 2, and T the maximal time of existence. Then for all t ∈
[0, T), we have

Z

S

u(t, x)dx= Z

S

u_{0}(x)dx,
Z

S

ρ(t, x)dx= Z

S

ρ_{0}(x)dx.

Proof. Integrating the first equation of (4.2) by parts, in view of the periodicity of uandG, we obtain

d dt

Z

S

udx=− Z

S

(u−γ)uxdx− Z

S

∂xG∗
u^{2}+1

2u^{2}_{x}+ (γ−A)u+1
2ρ^{2}

dx= 0.

On the other hand, integrating the second equation of (4.2) by parts, in view of the periodicity ofuandρ, we obtain

d dt

Z

S

ρdx=− Z

S

(uρ)xdx= 0.

Therefore, the proof is complete.

Lemma 4.7. Let (u, ρ)be the solution of system (4.1) with initial data(u0, ρ0)∈
H^{s}(S)×H^{s−1}(S), s≥ 2, and T the maximal time of existence. Then for all t ∈
[0, T), we have

Z

S

(u^{2}(t, x) +u^{2}_{x}(t, x) +ρ^{2}(t, x))dx=
Z

S

(u^{2}_{0}(t, x) +u^{2}_{0x}(t, x) +ρ^{2}_{0}(t, x))dx.

Proof. Multiplying the first equation of (4.1) by 2u and integrating by parts, we have

d dt

Z

S

(u^{2}(t, x) +u^{2}_{x}(t, x))dx= d
dt

Z

S

u_{x}(t, x)ρ^{2}(t, x)dx.

Multiplying the second equation of (4.1) by 2ρand integrating by parts, we obtain d

dt Z

S

ρ^{2}(t, x) =−d
dt

Z

S

ux(t, x)ρ^{2}(t, x)dx.

Adding the above two equalities, we obtain d

dt Z

S

(u^{2}(t, x) +u^{2}_{x}(t, x) +ρ^{2}(t, x))dx= 0.

This implies the desired result in this lemma.

Lemma 4.8 ([48]). For everyf ∈H^{1}(S), we have
max

x∈[0,1]f^{2}(x)≤ e+ 1

2(e−1)kfk^{2}_{H}1(S),
where the constant _{2(e−1)}^{e+1} is sharp.

By the conservation laws stated in Lemmas 4.6 and 4.7, we have the following corollary.

Corollary 4.9. Let(u, ρ)be the solution of system(4.1)with initial data(u_{0}, ρ_{0})∈
H^{s}(S)×H^{s−1}(S), s≥ 2, and T the maximal time of existence. Then for all t ∈
[0, T), we have

ku(t,·)k^{2}_{L}∞(S)≤ e+ 1

2(e−1)ku(t,·)k^{2}_{H}1(S)≤ e+ 1

2(e−1)k(u0, ρ_{0})k^{2}_{H}1(S)×L^{2}(S).
Lemma 4.10 ([25]). For all f ∈H^{1}(S), the following inequality holds

G∗(u^{2}+1

2u^{2}_{x})≥κu^{2}(x),
with

κ=1

2 + arctan (sinh(1/2))

2 sinh(1/2) + 2arctan (sinh(1/2)) sinh^{2}(1/2) ≈0.869.

Moreover,κis the optimal constant obtained by the function

f_{0}=1 + arctan (sinh(x−[x]−1/2)) sinh(x−[x]−1/2)
1 + arctan (sinh(1/2)) sinh(1/2) .

4.2. Blow-up scenario. Based on the above results, let us state the following theorem on the precise blow-up mechanism.

Theorem 4.11 (Wave-breaking criterion). Let (u, ρ)be the solution of (4.1)with
initial data(u0, ρ0)∈H^{s}(S)×H^{s−1}(S),s≥2, andT the maximal time of existence.

Then the solution blows up in finite time if and only if lim inf

t→T_{0}^{−}

{inf

x∈Su_{x}(t, x)}=−∞. (4.10)

To prove this wave-breaking criterion, we use the following lemma to show that indeeduxis uniformly bounded from above.

Lemma 4.12. Let(u, ρ)be the solution of (4.1)with initial data(u_{0}, ρ_{0})∈H^{s}(S)×

H^{s−1}(S), s≥2, andT the maximal time of existence. Then
sup

x∈S

u_{x}(t, x)≤ku_{0,x}k_{L}^{∞}+
q

kρ_{0}k^{2}_{L}∞+C_{1}^{2}. (4.11)
The constants above are defined as follows.

C0=k(u0, ρ0)k^{2}_{H}1×L^{2}, (4.12)
C_{1}^{2}=

(1−κ)e+ 1 e−1+1

2

C_{0}+(−1 + sinh 1)(γ−A)^{2}

4 sinh^{2}(1/2) , (4.13)
C2= 5e+ 3

4(e−1)C0+(−1 + sinh 1)(γ−A)^{2}

8 sinh^{2}(1/2) , (4.14)

andκis defined in Lemma 4.10.

Proof. The local well-posedness theorem and a density argument imply that it suffices to prove the desired estimates fors≥3. Thus, we takes= 3 in the proof.

Also, we assume thatu_{0}6≡0. Otherwise, the results become trivial. Differentiating
the first equation in (4.2) with respect tox. Using the identity−∂_{x}^{2}G∗f =f−G∗f,
we obtain

utx+(u−γ)uxx=−1

2u^{2}_{x}+u^{2}+1

2ρ^{2}−(γ−A)∂_{x}^{2}G∗u−G∗

u^{2}+1
2u^{2}_{x}+1

2ρ^{2}
. (4.15)
Using Lemma 4.1 and the fact that

sup

x∈S

(vx(t, x)) =−inf

x∈S

(−vx(t, x)), we can consider ¯m(t) andη(t) as follows,

η(t)∈S, m(t) :=¯ u_{x}(t, η(t)) = sup

x∈S

(u_{x}(t, x)), t∈[0, T). (4.16)
Hence,

uxx(t, η(t)) = 0, a.e. t∈[0, T).

For any x∈ S, take the trajectory q2(t, x) defined in (4.3). Then it follows from the second equation of (4.2) for the componentρthat

dρ(t, q2(t, x))

dt =−ux(t, q2(t, x))ρ(t, q2(t, x)). (4.17) It is known thatq2(t,·) :S→Sis a diffeomorphism for everyt∈[0, T). In view of Lemma 4.1, there existsx1(t)∈Ssuch that

q_{2}(t, x_{1}(t)) =η(t), t∈[0, T),

withη(0) =x1(0). Now define

ξ¯=ρ(t, η(t)), t∈[0, T).

Therefore, along the trajectoryq_{2}(t, x_{1}) =η(t), equations (4.15) and (4.17) become

¯

m^{0}(t) =−1
2m¯^{2}+1

2

ξ¯^{2}+f(t, η(t)),
ξ¯^{0}(t) =−ξ¯m,¯

(4.18)
fort∈[0, T), where “^{0}” denotes the derivative with respect to tandf(t, η(t)) is

f =u^{2}−(γ−A)∂_{x}^{2}G∗u−G∗(u^{2}+1
2u^{2}_{x}+1

2ρ^{2}).

We first derive the upper bound for f for later use in getting the wave-breaking results. Using Lemma 4.10 we have

f ≤(1−κ)u^{2}−(γ−A)∂xG∗∂xu, (4.19)
for anyx∈Sandt∈[0, T). Applying Young’s inequality withG=cosh(x−[x]−1/2)

2 sinh(1/2)

leads to

|γ−A||∂xG∗∂xu| ≤ |γ−A|kGxk_{L}2kuxk_{L}2 =|γ−A|

q1

2(−1 + sinh 1)
2 sinh(1/2) kuxk_{L}2

≤ (−1 + sinh 1)(γ−A)^{2}
8 sinh^{2}(1/2) +1

4kuxk^{2}_{L}2.

(4.20) Using Lemma 4.8, we obtain

u^{2}≤ ku(t,·)k^{2}_{L}∞(S)≤ e+ 1

2(e−1)k(u0, ρ_{0})k^{2}_{H}1×L^{2}. (4.21)
Therefore, in view of (4.20), (4.21) and the conservation law in Lemma 4.7, we
obtain the upper bound off for anyx∈Sandt∈[0, T),

f ≤ (1−κ)u^{2}+|γ−A||∂xG∗∂xu|

≤(1−κ) e+ 1

2(e−1)k(u_{0}, ρ_{0})k^{2}_{H}1×L^{2}+(−1 + sinh 1)(γ−A)^{2}
8 sinh^{2}(1/2) +1

4ku_{x}k^{2}_{L}2

≤

(1−κ) e+ 1 2(e−1)+1

4

k(u0, ρ0)k^{2}_{H}1×L^{2}+(−1 + sinh 1)(γ−A)^{2}
8 sinh^{2}(1/2)

= 1
2C_{1}^{2}.

(4.22)

Attention is now turned to the lower bound off. Similarly as before, we obtain
G∗ u^{2}+1

2u^{2}_{x}+1
2ρ^{2}

≤ kGk_{L}^{∞}ku^{2}+1
2u^{2}_{x}+1

2ρ^{2}k_{L}1

≤ cosh(1/2)

2 sinh(1/2)k(u0, ρ0)k^{2}_{H}1×L^{2}

= e+ 1

2(e−1)k(u0, ρ_{0})k^{2}_{H}1×L^{2}.

(4.23)

Using (4.20),(4.21) and (4.23), we have

−f ≤u^{2}+|γ−A||∂xG∗∂xu|+|G∗(u^{2}+1
2u^{2}_{x}+1

2ρ^{2})|

≤ e+ 1

e−1+1 4

k(u0, ρ0)k^{2}_{H}1×L^{2}+(−1 + sinh 1)(γ−A)^{2}
8 sinh^{2}(1/2)

= 5e+ 3

4(e−1)k(u0, ρ0)k^{2}_{H}1×L^{2}+(−1 + sinh 1)(γ−A)^{2}
8 sinh^{2}(1/2) .

(4.24)

Combining (4.22) and (4.24), we obtain

|f| ≤ 5e+ 3

4(e−1)k(u0, ρ_{0})k^{2}_{H}1×L^{2}+(−1 + sinh 1)(γ−A)^{2}

8 sinh^{2}(1/2) =C_{2}. (4.25)
Since nows≥3, we haveu∈C_{0}^{1}(S). Therefore,

inf

x∈Su_{x}(t, x)≤0, sup

x∈S

u_{x}(t, x)≥0, t∈[0, T).

Hence, ¯m(t)≥0 fort∈[0, T). From the second equation of (4.18), we obtain that
ξ(t) = ¯¯ ξ(0)e^{−}^{R}^{0}^{t}^{m(τ)dτ}^{¯} .

Hence,

|ρ(t, η(t))|=|ξ(t)| ≤ |¯ ξ(0)| ≤ |ρ¯ 0(x1(0))| ≤ kρ0kL^{∞}.
Now define

P1(t) = ¯m(t)− ku0,xkL^{∞}−q

kρ0k^{2}_{L}∞+C_{1}^{2}.
Note thatP1(t) is aC^{1}−differentiable function in [0, T) and satisfies

P1(0)≤m(0)¯ − ku0,xkL^{∞}≤0.

We will show that

P1(t)≤0, t∈[0, T). (4.26)

If not, then suppose there is at0∈[0, T) such thatP1(t0)>0. Define t1= max{t < t0:P1(t) = 0}.

ThenP1(t1) = 0 andP_{1}^{0} ≥0, or equivalently,

¯

m(t1) =ku0,xkL^{∞}+
q

kρ0k^{2}_{L}∞+C_{1}^{2},

¯

m^{0}(t1)≥0.

On the other hand, we have

¯

m^{0}(t1) =−1

2m¯^{2}(t1) +1
2

ξ¯^{2}(t1) +f(t1, η(t1))

≤ −1 2

ku0,xkL^{∞}+
q

kρ0k^{2}_{L}∞+C_{1}^{2}
^{2}

+1

2kρ0k^{2}_{L}∞+C_{1}^{2}
2 <0,
which is a contradiction. Therefore,P1(t)≤0, fort∈[0, T), and we obtain (4.26).

Therefore, the proof is complete.

It is also found that ifu_{x} is bounded from below, we may obtain the following
estimates forkρkL^{∞}(S).

Lemma 4.13. Let(u, ρ)be the solution of (4.1)with initial data(u0, ρ0)∈H^{s}(S)×

H^{s−1}(S), s≥2, andT the maximal time of existence. If there is anM ≥0, such
that

inf

(t,x)∈[0,T)×S

u_{x}≥ −M, (4.27)

then

kρ(t,·)k_{L}^{∞}_{(}_{S}_{)}≤ |ρ0k_{L}^{∞}_{(}_{S}_{)}e^{M t}. (4.28)
Proof. For any givex∈S, we define

U(t) =ux(t, q2(t, x)), γ(t) =ρ(t, q2(t, x)),

with q2(t, x(t)) =x, for somex(t)∈R, t ∈[0, T). Then theρequation of system (4.1) becomes

γ^{0}=−γU.

Thus,

γ(t) =γ(0)e^{−}^{R}^{0}^{t}^{U(τ)dτ}.
From assumption (4.27), we see that

U(t)≥ −M, t∈[0, T).

Hence,

|ρ(t, q_{2}(t, x(t))|=|γ(t)| ≤ |γ(0)|e^{−}^{R}^{0}^{t}^{U}^{(τ)dτ}≤ kρ_{0}k_{L}^{∞}e^{M t},

which together with (4.5) leads to (4.28).

We are now in the position to prove Theorem 4.11.

Proof of Theorem 4.11. Assume thatT <∞and (4.10) is not valid. Then there is some positive numberM >0 such that

u_{x}(t, x)≥ −M, ∀(t, x)∈[0, T)×S.
It is now inferred from Lemma 4.12 that|ux(t, x)| ≤C, where

C=C(A, γ, M,k(u0, ρ0)k^{2}_{H}s×H^{s−1}).

Therefore, Theorem 4.5 in turn implies that the maximal existence time T =∞,
which contradicts the assumption thatT <∞. Conversely, the Sobolev embedding
theoremH^{s}(S),→L^{∞}(S) withs >1/2 implies that if (4.10) holds, the correspond-
ing solution blows up in finite time. This completes the proof.

Now, we give the following theorems with some initial conditions which guarantee wave breaking in finite time.

Theorem 4.14. Let (u, ρ)be the solution of (4.1)with the initial data(u0, ρ0)∈
H^{s}(S)×H^{s−1}(S), s≥2, andT the maximal time of existence. Assume that there
is somex_{0}∈Ssuch that

ρ0(x0) = 0, u0,x(x0) = inf

x∈S

u0,x(x), and

u0,x(x0)<−C1, (4.29)

whereC1 is defined as
C_{1}^{2}=

(1−κ)e+ 1 e−1+1

2

k(u0, ρ0)k^{2}_{H}1×L^{2}+(−1 + sinh 1)(γ−A)^{2}
4 sinh^{2}(1/2) .

Then the corresponding solution to system (4.1) blows up in the following sense:

there exists aT1 with

0< T1≤ − 2 u0,x(x0) +p

−C1u0,x(x0) (4.30) such that

lim inf

t→T_{0}^{−}

{inf

x∈Su_{x}(t, x)}=−∞.

Proof. Similar to the proof of Lemma 4.12, it suffices to considers≥3. So in the following of this sections= 3 is taken for simplicity of notation.

we consider the functionsm(t) andξ(t)∈Sas in Lemma 4.12
m(t) :=u_{x}(t, ξ(t)) = inf

x∈S(u_{x}(t, x)), t∈[0, T).

Hence,

uxx(t, ξ(t)) = 0, a.e. t∈[0, T). (4.31)
Similar as before, we can choosex_{2}(t)∈S such that

q2(t, x2(t)) =ξ(t) t∈[0, T).

Along the trajectory ofq2(t, x), we have dρ(t, ξ(t))

dt =−ρ(t, ξ(t))ux(t, ξ(t)).

It follows from the assumption of the theorem, that m(0) =ux(0, ξ(0)) = inf

x∈S

u0,x(x) =u0,x(x0).

Hence, we can chooseξ(0) =x_{0} and thenρ_{0}(ξ(0)) =ρ_{0}(x_{0}) = 0. Thus, from (4.8)
we obtain

ρ(t, ξ(t)) = 0, t∈[0, T). (4.32) Differentiating the first equation in (4.2) with respect tox, evaluating the result at x=ξ(t) and using (4.31) and (4.32), we deduce from (4.15) that

m^{0}(t) =−1

2m^{2}(t) +f(t, ξ(t)). (4.33)
Using the upper bound off in (4.22), it is found that

m^{0}(t)≤ −1

2m^{2}(t) +1

2C_{1}^{2}, a.e. t∈[0, T).

By assumption (4.29),m(0) =u_{0,x}(x_{0})<−C1, we deduce thatm^{0}(0)<0 andm(t)
is strictly decreasing over [0, T). Set

δ= 1 2−1

2

s C_{1}

−u0,x(x0) ∈

0,1 2

. (4.34)

Using thatm(t)< m(0) =u0,x(x0)<0, it follows that
m^{0}(t)≤ −1

2m^{2}(t) +1

2C_{1}^{2}≤ −δm^{2}(t), a.e. t∈[0, T). (4.35)
Integrating on both sides in (4.35), it is inferred that

m(t)≤ u0,x(x0)

1 +δu0,x(x0)t → −∞ as t→ − 1

δu0,x(x0). (4.36)

Hence,

T ≤ − 1

δu0,x(x0), (4.37)

which proves (4.30).

Corollary 4.15. With the assumptions of Theorem 4.14, assumes >5/2. There
exists aT^{∗} with0< T1≤T^{∗}, (T1 is defined in (4.30)) such that

(a) lim sup_{t→T}∗{sup_{x∈}_{S}ρx(t, x)}=∞, if ρ0,x(x0)>0,
(b) lim inf_{t→T}^{∗}{inf_{x∈S}ρx(t, x)}=−∞, if ρ0,x(x0)<0.

Proof. With the assumptions of Theorem 4.14, we have ρ0(x0) = 0, u0,x(x0) = inf

x∈S

u0,x(x),

andu_{0,x}(x_{0})<−C_{1}. Evaluating ρalong the trajectory q_{2}(t, x), we obtain
dρ_{x}(t, q_{2}(t, x))

dt =−uxx(t, q_{2}(t, x))ρ(t, q_{2}(t, x))−2u_{x}(t, q_{2}(t, x))ρ_{x}(t, q_{2}(t, x)).

As in the proof of Theorem 4.14, we can choosex_{2}(t)∈S such thatq_{2}(t, x_{2}(t)) =
ξ(t), t∈[0, T). Then we have

m(t) :=u_{x}(t, ξ(t)) = inf

x∈S

(u_{x}(t, x)), t∈[0, T).

Hence,uxx(t, ξ(t)) = 0, a.e. t∈[0, T). This in turn implies
dρ_{x}(t, ξ(t))

dt =−2ux(t, ξ(t))ρx(t, ξ(t)), and

ρx(t, ξ(t)) =ρ0,x(x0)e^{−2}^{R}^{0}^{t}^{u}^{x}^{(τ,ξ(τ))dτ} =ρ0,x(x0)e^{−2}

Rt 0 inf

x∈Su_{x}(τ,x)dτ

. Sincem(t) is strictly decreasing in [0, T), by (4.36) we have

e^{−2}

Rt 0 inf

x∈Su_{x}(τ,x)dτ

≥e^{−2}

Rt 0

u0,x(x0 ) 1+δu0,x(x0 )τdτ

≥e^{−}^{2}^{δ}^{ln(1+δu}^{0,x}^{(x}^{0}^{)t)},
whereδis defined in (4.34). So

e^{−}^{2}^{δ}^{ln(1+δu}^{0,x}^{(x}^{0}^{)t)}→+∞,
if t → −_{δu} ^{1}

0,x(x_{0}). Therefore, it is inferred from (4.37) that there exists someT^{∗}
with 0< T1≤T^{∗}such that

sup

x∈S

ρx(t, x)≥ρx(t, ξ(t))→+∞.

as t →T^{∗}. Ifρ_{0,x}(x_{0})<0, the proof is similar to the above. This completes the

proof of the corollary.

Theorem 4.16. Let (u, ρ)be the solution of (4.1)with the initial data(u0, ρ0)∈
H^{s}(S)×H^{s−1}(S), s≥2, and T the maximal time of existence. Also assume that
R

Sρ0(x)dx= 0, andkρx(t,·)kL^{∞}(S)≤M (M is a positive constant). If there exists
someK0=K0(C0)>0 (C0=k(u0, ρ0)k^{2}_{H}1×L^{2}) such that

Z

S

u^{3}_{0x}dx <−K0, (4.38)

then the corresponding solution to (4.1)blows up in finite time.

Proof. Applyingu^{2}_{x}∂xto both sides of the first equation in (4.2) and integrating by
parts with the fact that

−3 Z

S

uu^{2}_{x}uxxdx=
Z

S

u^{4}_{x}dx.

We have d dt

Z

S

u^{3}_{x}dx+1
2

Z

S

u^{4}_{x}dx= 3
Z

S

u^{2}_{x}

u^{2}+ (γ−A)u+1
2ρ^{2}

dx

−3 Z

S

u^{2}_{x}G∗
u^{2}+1

2u^{2}_{x}+ (γ−A)u+1
2ρ^{2}

dx.

(4.39)

Note that

Z

S

u^{3}_{x}dx
≤Z

S

u^{4}_{x}dx1/2Z

S

u^{2}_{x}dx1/2

,
andC_{0}=k(u_{0}, ρ_{0})k^{2}_{H}1×L^{2}. Thus we have

Z

S

u^{4}_{x}dx≥ 1
C0

Z

S

u^{3}_{x}dx^{2}

. (4.40)

Using Corollary 4.9, we obtain the estimate Z

S

u^{2}_{x}u^{2}dx≤ kuk^{2}_{L}∞(S)

Z

S

u^{2}_{x}dx≤ e+ 1

2(e−1)C_{0}^{2}. (4.41)
By the assumptionR

Sρ0(x)dx= 0 and Lemma 4.2, we have Z

S

ρ(t, x)dx= Z

S

ρ_{0}(x)dx= 0.

It then follows that for anyt∈[0, T), there existsx3(t)∈Sandρ(t, x3(t)) = 0. It is noted that

ρ(t, x) = Z x(t)

x_{3}(t)

ρx(t, s)ds, x3(t), x(t)∈S, which implies that

|ρ(t, x)| ≤

Z x(t)

x3(t)

ρx(t, s)ds ≤M, Z

S

u^{2}_{x}ρ^{2}dx≤M^{2}
Z

S

u^{2}_{x}dx≤M^{2}C_{0}, (4.42)

Z

S

u^{2}_{x}udx

≤ kuk_{L}∞(S)

Z

S

u^{2}_{x}dx≤ e+ 1
2(e−1)

^{1/2}

C_{0}^{3/2}, (4.43)
and

Z

S

u^{2}_{x}G∗(γ−A)udx

≥ −|γ−A|kGkL^{∞}(S)kukL^{∞}(S)

Z

S

u^{2}_{x}dx

≥ −|γ−A| cosh(1/2) 2 sinh(1/2)

e+ 1 2(e−1)

^{1/2}

C_{0}^{3/2}=−|γ−A| e+ 1
2(e−1)

^{3/2}
C_{0}^{3/2}.

(4.44)