Open Access

Global solutions to a class of nonlinear damped wave operator equations

Boundary Value Problems20122012:42

DOI: 10.1186/1687-2770-2012-42

Received: 6 October 2011

Accepted: 13 April 2012

Published: 13 April 2012

Abstract

This study investigates the existence of global solutions to a class of nonlinear damped wave operator equations. Dividing the differential operator into two parts, variational and non-variational structure, we obtain the existence, uniformly bounded and regularity of solutions.

Mathematics Subject Classification 2000: 35L05; 35A01; 35L35.

Keywords

nonlinear damped wave operator equations global solutions uniformly bounded regularity

1 Introduction

In recent years, there have been extensive studies on well-posedness of the following nonlinear variational wave equation with general data:
t 2 u - c ( u ) x c ( u ) x u = 0 in ( 0 , ) × R , u | t = 0 = u 0 on R , t u | t = 0 = u 1 on R ,
(1.1)
where c(·) is given smooth, bounded, and positive function with c'(·) ≥ 0 and c'(u0) > 0,u0 H1(R),u1(x) L2(R). Equation (1.1) appears naturally in the study for liquid crystals [14]. In addition, Chang et al. [5], Su [6] and Kian [7] discussed globally Lipschitz continuous solutions to a class one dimension quasilinear wave equations
u t t - p ρ ( x ) , u x x = ρ ( x ) h ρ ( x ) , u , u x , u ( x , 0 ) = u 0 ( x ) , u t ( x , 0 ) = ω 0 ( x ) ,
(1.2)
where (x,t) R × R+, u0(x),ω0(x) R. Furthermore, Nishihara [8] and Hayashi [9] obtained the global solution to one dimension semilinear damped wave equation
u t t + u t - u x x = f ( u ) , ( t , x ) R + × R + ( u , u t ) ( 0 , x ) = ( u 0 , u 1 ) ( x ) .
(1.3)

Ikehata [10] and Vitillaro [11] proved global existence of solutions for semilinear damped wave equations in R N with noncompactly supported initial data or in the energy space, in where the nonlinear term f(u) = |u| p or f(u) = 0 is too special; some authors [1214] discussed the regularity of invariant sets in semilinear wave equation, but they didn't refer to any the initial value condition of it. Unfortunately, it is difficulty to classify a class wave operator equations, since the differential operator structure is too complex to identify whether have variational property. Our aim is to classify a class of nonlinear damped wave operator equations in order to research them more extensively and go beyond the results of [12].

In this article, we are interested in the existence of global solutions of the following nonlinear damped wave operator equations:
d 2 u d t 2 + k d u d t = G ( u ) , k > 0 u ( x , 0 ) = φ ( x ) , u t ( x , 0 ) = ψ ( x ) ,
(1.4)

where G : X 2 × R + X 1 * is a mapping, X2 X1, X1, X2 are Banach spaces and X 1 * is the dual spaces of X1, R+ = [0, ∞), u = u(x,t). If k > 0, (1.4) is called damped wave equation. We obtain the existence, uniformly bounded and regularity of solutions by dividing the differential operator G(u) into two parts, variational and non-variational structure.

2 Preliminaries

First we introduce a sequence of function spaces:
X H 2 X 2 X 1 H , X 2 H 1 H ,
(2.1)
where H, H1, H2 are Hilbert spaces, X is a linear space, X1, X2 are Banach spaces and all inclusions are dense embeddings. Suppose that
L : X X 1 is one to one dense linear operator , L u , v H = u , v H , u , v X .
(2.2)
In addition, the operator L has an eigenvalue sequence
L e k = λ k e k , ( k = 1 , 2 , . . . )
(2.3)
such that {e k } X is the common orthogonal basis of H and H2. We investigate the existence of global solutions of the Equation (1.4), so we need define its solution. Firstly, in Banach space X, introduce
L p ( ( 0 , T ) , X ) = u : ( 0 , T ) X | 0 T u p d t < ,
where p = (p1, p2,..., p m ),p i ≥ 1(1 ≤ im),
u p = k = 1 m u k p k ,
where | · | k is semi-norm in X, and X = i = 1 m i . Similarily, we can define
W 1 , p ( ( 0 , T ) , X ) = u : ( 0 , T ) X | u , u L p ( ( 0 , T ) , X ) .

Let L loc p ( ( 0 , ) , X ) = u ( t ) X | u L p ( ( 0 , T ) , X ) , T > 0 . .

Definition 2.1. Set (φ, ψ) X2 × H1, u W loc 1 , 0 , , H 1 L loc ( ( 0 , ) , X 2 ) is called a globally weak solution of (1.4), if for v X1, it has
u t , v H + k u , v H = 0 t G u , v d t + k φ , v H + ψ , v H .
(2.4)
Definition 2.2. Let Y1,Y2 be Banach spaces, the solution u(t, φ, ψ) of (1.4) is called uniformly bounded in Y1 × Y2, if for any bounded domain Ω1 × Ω2Y1 × Y2, there exists a constant C which only depends the domain Ω1 × Ω2, such that
u Y 1 + u t Y 2 C , ( φ , ψ ) Ω 1 × Ω 2 and t 0 .
Definition 2.3. A mapping G : X 2 X 1 * is called weakly continuous, if for any sequence {u n } X2, u n u0 in X2,
lim n G ( u n ) , v = G ( u 0 ) , v , v X 1 .

Lemma 2.1. [15]Let H2, H be Hilbert spaces, and H2 H be a continuous embedding. Then there exists a orthonormal basis {e k } of H, and also is one orthogonal basis of H2.

Proof. Let I : H2H be imbedded. According to assume I is a linear compact operator, we define the mapping A : H2H as follows
A u , v H 2 = I u , v H = u , v H , v H 2 .
obviously, A : H2H2 is linear symmetrical compact operator and positive definite. Therefore, A has a complete eigenvalue sequence {λ k } and eigenvector sequence k H 2 such that
A k = λ k k , k = 1 , 2 , . . . ,
and k is orthogonal basis of H2. Hence
i , j H = A i , j H 2 = λ i i , j H 2 = 0 , if i j .

it implies i is also orthogonal sequence of H. Since H2 H is dense, i is also orthogonal sequence of H, so { e i } = i / i H is norm orthogonal basis of H. The proof is completed.

Now, we introduce an important inequality

Lemma 2.2. [16] (Gronwall inequality) Let x(t), y(t), z(t) be real function on [a, b], where x(t) ≥ 0,atb, z(t) C[a, b], y(t) is differentiable on [a, b]. If the inequality as follows is hold
z ( t ) y ( t ) + a t x ( τ ) z ( τ ) d τ , a t b ,
(2.5)
then
z ( t ) y ( a ) e a t x ( s ) d s + a t e a t x ( τ ) d y d s d s .
(2.6)

3 Main results

Suppose that G = A + B : X 2 × R + X 1 * . Throughout of this article, we assume that
  1. (i)
    There exists a function F C 1 : X 2R 1 such that
    A u , L v = - D F ( u ) , v , u , v X
    (3.1)
     
  2. (ii)
    Function F is coercive, if
    F ( u ) u X 2
    (3.2)
     
  3. (iii)
    B as follows
    B u , L v C 1 F ( u ) + C 2 v H 1 2 , u , v X
    (3.3)
     

for some g L loc 1 ( 0 , ) .

Theorem 3.1. Set G : X 2 × R + X 1 * is weakly continuous, (φ, ψ) X2 × H1, then we obtain the results as follows:

(1) If G = A satisfies the assumption (i) and (ii), then there exists a globally weak solution of (1.4)
u W loc 1 , ( 0 , ) , H 1 L loc ( ( 0 , ) , X 2 )

and u is uniformly bounded in X2 × H1;

(2) If G = A + B satisfies the assumption (i), (ii) and (iii), then there exists a globally weak solution of (1.4)
u W loc 1 , ( ( 0 , ) , H 1 ) L loc ( ( 0 , ) , X 2 ) ;
(3) Furthermore, if G = A + B satisfies
G u , v 1 2 v H 2 + C F ( u ) + g ( t )
(3.4)

for some g L loc 1 ( 0 , ) , then u W loc 2 , 2 ( 0 , ) , H .

Proof. Let {e k } X be the public orthogonal basis of H and H2, satisfies (2.3).

Note
X n = i = 1 n α i e i | α i R 1 , X ̃ n = j = 1 n β j ( t ) e j | β j C 2 0 , .
(3.5)
From the assumption, we know L X n = X n , L X n ̃ = X n ̃ , apply the Galerkin method to make truncate in X n ̃ :
d 2 u i d t 2 + k d u i d t = G ( u n ) , e i , 1 i n u i ( x , 0 ) = φ , e i H , u i ( x , 0 ) = ψ , e i H
(3.6)
there exists u n = i = 1 n u i ( t ) e i C 2 ( 0 , ) , X n for any v X n ̃ satisfies
0 t d 2 u n d t 2 + k d u n d t , v H d t = 0 t G u n , v d t
(3.7)
for any v X n , it yields that
d u n d t , v H + k u n , v H = 0 t G u n , v d t + k φ , v H + ψ , v H
(3.8)
  1. (1)
    If G = A , u n X n ̃ substitute v = d d t L u n into (3.7), we get
    0 t d 2 u n d t 2 + k d u n d t , d d t L u n H 1 d t = 0 t G u n , d d t L u n d t
     
combine condition (2.2) with (3.1), we get
0 t Ω d 2 u n d t 2 d u n d t d x d t + 0 t Ω k d u n d t d u n d t d x d t + 0 t D F ( u n ) d u n d t d x d t = 0 0 t 1 2 d d t d u n d t H 1 2 d t + k 0 t d u n d t H 1 2 d t + 0 t d d t F ( u n ) d t = 0 1 2 d u n d t H 1 2 - 1 2 ψ n H 1 2 + k 0 t d u n d t H 1 2 d t + F ( u n ) - F ( φ n ) = 0
consequently, we get
F ( u n ) + 1 2 u n H 1 2 + k 0 t u n H 1 2 d t = F ( u n ) + 1 2 ψ n H 1 2 .
(3.9)
Assume φ H2, combine(2.2)with(2.3), we know {e n } is also the orthogonal basis of H1, then φ n φ in H2, ψ n ψ in H1, owing to H2 X2 is embedded, so
φ n φ i n X 2 ψ n ψ i n X 1
(3.10)
due to the condition (3.6), from (3.9)and (3.10) we easily know
{ u n } W loc 1 , ( ( 0 , ) , H 1 ) L loc ( ( 0 , ) , X 2 ) i s b o u n d e d .
consequently, assume that
u n u 0 i n W loc 1 , ( ( 0 , ) , H 1 ) L loc ( ( 0 , ) , X 2 ) a . e . t > 0
i.e. u n u0in X2a.e. t > 0, and G is weakly continuous, so
lim n G u n , v = G u 0 , v .
By (3.8), we have
lim n d u n d t H + k u n , v H = lim n 0 t G u n , v d t + k φ , v H + ψ , v H d u 0 d t , v H + k u 0 , v H = 0 t G u 0 , v d t + k φ , v H + ψ , v H
it indicates for any v n = 1 X n X 2 , it holds. Hence, for any v X2, we have
d u 0 d t , v H + k u 0 , v H = 0 t G u 0 , v d t + k φ , v H + ψ , v H .
(3.11)

Consequently, u0 is a globally weak solution of (1.4).

Furthermore, by (3.9) and (3.10), for any R > 0, there exists a constant C such that if
φ X 2 + ψ H 1 R
(3.12)
then the weak solution u(t, φ, ψ) of (1.4) satisfies
u ( t , φ , ψ ) X 2 + u t ( t , φ , ψ ) H 1 C . t 0
(3.13)
Assume (φ,ψ) X2 × H1 satisfies (3.12), by H2 X2 is dense. May fix φ n H2 such that
φ n X 2 + ψ H 1 R , lim n φ n = φ in X 2

by (3.13), the solution {u(t, φ n , ψ)} of (1.4) is bounded in W loc 1 , ( 0 , ) , H 1 L loc ( ( 0 , ) , X 2 ) a.e. t > 0.

Therefore, assume u(t, φ n , ψ) u in W loc 1 , ( 0 , ) , H 1 L loc ( ( 0 , ) , X 2 ) then u(t) is a weak solution of (1.4), it satisfies uniformly bounded of (3.13). So the conclusion (1) is proved.
  1. (2)
    If G = A + B , u n X n ̃ , substitute v = d d t L u n into (3.7), we get
    0 t d 2 u n d t 2 , d d t L u n H 1 + k d u n d t , d d t L u n 1 H 1 d t = 0 t A u n , d u n d t + B u n , d u n d t d t
     
combine the condition (2.2) and (3.1), we have
0 t Ω d 2 u n d t 2 d u n d t d x d t + k 0 t Ω d u n d t 2 d u n d t d x d t + 0 t D F ( u n ) d u n d t d t = 0 t B u n , d u n d t d t 0 t 1 2 d d t d u n d t H 1 2 d t + k 0 t d u n d t H 1 2 d t + 0 t d d t F ( u n ) d t = 0 t B u n , d u n d t d t 1 2 u n H 1 2 - 1 2 ψ n H 1 2 + k 0 t u n H 1 2 d t + F ( u n ) + F ( φ n ) = 0 t B u n , d u n d t d t
consequently, we have
F ( u n ) + 1 2 u n H 1 2 + k 0 t u n H 1 2 d t = 0 t B u n , d u n d t d t + F ( φ n ) + 1 2 ψ n H 1 2
(3.14)
by the condition (3.3),(3.14)implies
F ( u n ) + 1 2 u n H 1 2 C 0 t F ( u n ) + 1 2 u n H 1 2 d t + f ( t )
(3.15)

where f ( t ) = 0 t g ( τ ) d t + 1 2 ψ H 1 2 + sup n F ( φ n ) . .

by Gronwall inequality [Lemma(2.2)], from (3.15) we easily know:
F ( u n ) + 1 2 u n H 1 2 f ( 0 ) e C t + 0 t f ( τ ) e C ( t - τ ) d τ
(3.16)
it implies that, for any 0 < T < ∞
{ u n } W 1 , ( 0 , T ) , X 2 L 0 , T , X 2 is bounded .
now, use the same way as (1), we can obtain the result (2).
  1. (3)
    If the condition (3.4) is hold, u n X n ̃ , substitute v = d 2 u d t 2 into (3.7), we can get
    0 t d 2 u n d t 2 , d 2 u n d t 2 H + k d u n d t , d 2 u n d t 2 H d t = 0 t G u n , d 2 u n d t 2 d t
     
then
0 t d 2 u n d t 2 , d 2 u n d t 2 H d t + k 2 0 t d d t u n ( t ) H 2 d t 0 t 1 2 u n ( t ) H 2 + C F ( u n ) + g ( t ) d t 0 t d 2 u n d t 2 , d 2 u n d t 2 H d t + k 2 u n H 2 k 2 ψ n H 2 + 0 t 1 2 d 2 u n d t 2 H 2 + C F ( u n ) + g ( τ ) d τ
by (3.16), it implies that
0 t d 2 u n d t 2 H 2 d τ C , ( C > 0 )
consequently, for any 0 < T < ∞
{ u n } W 2 , 2 ( ( 0 , T ) , H ) is bounded .

it implies that u W2,2((0,T), H), the main theorem (3.1) has been proved.

Declarations

Acknowledgements

The author is very grateful to the anonymous referees whose careful reading of the manuscript and valuable comments enhanced presentation of the manuscript. Foundation item: the National Natural Science Foundation of China (No. 10971148).

Authors’ Affiliations

(1)
Yangtze Center of Mathematics, Sichuan University
(2)
College of Mathematics and Software Science, Sichuan Normal University

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© Pan et al; licensee Springer. 2012

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