BLOW-UP OF SOLUTIONS FOR A NONLINEAR WAVE EQUATION WITH NONNEGATIVE INITIAL ENERGY

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1 Electronic Journal of Differential Equations, Vol. 213 (213, No. 115, pp ISSN: URL: or ftp ejde.math.txstate.edu BLOW-UP OF SOLUTIONS FOR A NONLINEAR WAVE EQUATION WITH NONNEGATIVE INITIAL ENERGY WENJUN LIU, YUN SUN, GANG LI Abstract. In this article, we study a wave equation with nonlinear boundary damping and interior source term. We prove two blow-up results with nonnegative initial energy; thus we extend the blow-up results by Feng et al [5]. 1. Introduction In this article, we study the following wave equation with nonlinear boundary damping and interior source term y tt (x, t y xx (x, t = y(x, t p 1 y(x, t, (x, t (, L (, T, y(, t =, y x (L, t = y t (L, t m 1 y t (L, t, t [, T, y(x, = y (x, y t (x, = y 1 (x, x [, L], (1.1 where (, L is a bounded open interval in R, m > 1, p > 1. The wave equation with interior damping term has been extensively studied and several results concerning existence, asymptotic behavior and blow-up have been established. When m = 1, Levine [7, 8] proved that the solution blows up in finite time with negative initial energy. When m > 1, Georgiev and Todorova [6] extended this result and established a global existence result if m p and a blow-up result if m < p for sufficiently large initial data. Later Messaoudi [12] improved [6] by considering only negative initial energy. The wave equation with boundary source term has also been extensively studied. Vitillaro [15] proved the existence of a global solution when p m or the initial data are inside the potential well. In [19], Zhang and Hu proved the decay result when the initial data are inside a stable set, and the blowup result when p > m and the initial data is inside an unstable set. For other wave equations with nonlinear source and damping terms, we can also refer the reader to [1, 2, 3, 4, 1, 11, 13, 16, 17, 18] and references therein. Recently, Feng et al [5] considered (1.1 and obtained the blow-up results with one of the following conditions: (A 2m < and E( < ; (B 2m, 4p (p 1( E( <, and L >. Later, Li et al [9] studied the interaction between the interior damping y t (x, t and the boundary source y(l, t p 1 y(l, t + by(l, t and 2 Mathematics Subject Classification. 35B44, 35L1, 35L71. Key words and phrases. Boundary damping term; interior source term; nonlinear wave equation; nonnegative initial energy. c 213 Texas State University - San Marcos. Submitted January 18, 213. Published May 6,

2 2 W. LIU, Y. SUN, G. LI EJDE-213/115 established three sufficient conditions for the blow-up results with some necessary restriction on b when the initial energy is positive or negative. Motivated by [9], we intend to extend the results in [5] with nonnegative initial energy. For this purpose, we use an improved relationship between E 1 and y which is given in Lemma 3.1 below. This article is organized as follows. In Section 2, we present some notation needed for our work and state our main results. In Section 3, we give the proof of Theorem 2.2. Section 4 is devoted to the proof Theorem 2.3. We define the following functionals 2. Notation and main results E(t = 1 2 y t(t y x(t y(t, (2.1 I(t = y x (t 2 2 y(t, (2.2 and as in [5] we introduce the notation: q = L q (,L and the Hilbert space H 1 left(, L := {u H 1 (, L : u( = }. (2.3 Set ( 1 E 1 := 2 1 α, α := C 2( p 1, (2.4 where C is the optimal constant of the Sobolev embedding y C y x 2, for any y Hleft 1 (, L. Next, we give a the existence of a local solution. Theorem 2.1 ([5, Theorem 2.1]. Assume that (y, y 1 H 1 left (, L L2 (, L. Then (1.1 has a unique local solution y(x, t satisfying y(x, t C(, T m ; H 1 left(, L, y t (L, t L m+1 (, T m for some T m >, and the energy equality holds for t < T m. E(t + t Our main results are as follows. y t (x, t C(, T m ; L 2 (, L, y t (L, τ m+1 dτ = E( (2.5 Theorem 2.2. Let y(x, t be a solution of problem (1.1. Assume that 2m <, I( < and for any fixed < θ < 1, E( < θe 1. Then the solution blows up in finite time. Theorem 2.3. Let y(x, t be a solution of problem (1.1. Assume that 2m, I( < and for any fixed < θ < 1, E( < θe 1. Furthermore, we assume that L > 4p + 2p(p 1 θ(p 1 2 (p 1([1 θ(p 1 then the solution blows up in finite time. ], (2.6 Remark 2.4. When E( <, the blow-up results have been proved in [5]. So, we consider here only the case E(.

3 EJDE-213/115 BLOW-UP OF SOLUTIONS 3 Remark 2.5. In the case 2m, we note that the similar restriction on L as (2.6 has been used in [5], which means that the larger the interval (, L is, the less the boundary damping effect. It is still the case when I( < and E( < θe Proof of Theorem 2.2 In this section, we consider the blow-up result in the case 2m <. For this purpose, we give the following lemmas first. Lemma 3.1. Let y(x, t be a solution of problem (1.1 with E( < θe 1 and I( <. Then there exists a positive constant < β < 1 such that E 1 < β p 1 y(x, t, t >. (3.1 2( Proof. We adopt the manner which was first introduced in [14]. From (2.1 and Sobolev embedding, we have E(t 1 2 y x y 1 2 y x 2 2 C y x Let h(ξ = 1 2 ξ C ξ 2, then E(t h(ξ with ξ = y x 2 2. It is easy to see that h(ξ is strictly increasing on [, α, strictly decreasing on (α, + and takes its maximum value E 1 at α. Since I( <, we have which leads to Furthermore, since yx 2 2 < y C yx 2, y x 2 2 > α, for α defined by (2.4. E 1 > E( E(t h( y x 2 2, t, there exists no time t such that y x (t 2 2 = α. By the continuity of y x 2 2, we obtain y x 2 2 > α, t. On the other hand, we have 1 y E( y t y x 2 2 > θe α = which gives E 1 < p 1 2 2( ( θ(p 1 y 2 Taking β = ( θ(p 1 (, 1, inequality (3.1 follows. Set H(t = θe 1 E(t,. then it is clear that H(t is increasing, H(t H( > and H(t 2. ( p 1 θ E 1, θβ(p y 2(. (3.2

4 4 W. LIU, Y. SUN, G. LI EJDE-213/115 Lemma 3.2. Under the assumptions of Lemma 3.1, there exists a positive constant C such that y s C y, (3.3 for any 2 s. Proof. If y 1, then y s C y, since s. If y < 1, then y s y 2, since 2 s. Using the Sobolev embedding inequality, (2.1, and Lemma 3.1, we have ( ( y 2 C y x 2 2 2C E(t + y 2C This completes the proof. As in [5], we choose a constant r such that Then we infer that E 1 + y C y. (3.4 < max { 2, m } < r < 1. (3.5 m 2 m + 1, m r + 1 r, (1 + r <. (3.6 2 Lemma 3.3. Under the assumptions of Lemma 3.1, there exists a positive constant C such that y(l, t m+1 C [ y m+1 r+1 r + y m + y 2 (1+r ]. (3.7 Proof. By using Lemme 3.2 and the proof of [5, Lemma 3.2], we obtain (3.7. Set L(t = H 1 σ (t + ε y(ty t dx, (3.8 for ε small to be chosen later and < σ < min { p 1 2(, p m m(, 1 m 1 + r r(, 1 m 1 + r }. (3.9 2m Then we have the following lemma. Lemma 3.4. Under the assumptions of Lemma 3.1, there exists a positive constant C such that H σm (t y(l, t m+1 C y, (3.1 for any 2m <. Proof. By using (3.2, Lemma 3.3, Lemma 3.2 and the proof of [5, Lemma 3.3], we complete the proof. Now, we are ready to proof our first main result. Proof of Theorem 2.2. Computing a derivative of (3.8 yields L (t (1 σh σ (t y t (L, t m+1 + 2ε y t (t εH(t 2εθE 1 ε y t (L, t m y(l, t + ε p 1 y(t. (3.11

5 EJDE-213/115 BLOW-UP OF SOLUTIONS 5 Using Young s inequality and (3.1, we have L (t (1 σh σ (t y t (L, t m+1 + 2ε y t (t εH(t ε(p 1(1 θβ + y(t mε m+1 m + 1 δ m yt (L, t m+1 ε m + 1 δm+1 y(l, t m+1. (3.12 Let δ m+1 = k m H σm for k > to be chosen later, then from (3.12 and Lemma 3.4 we obtain L (t ( 1 σ kmε H σ (t y t (L, t m+1 + 2ε y t (t 2 2 m ε [ (p 1(1 θβ Choose k large enough so that then (3.13 reduces to L (t (p 1(1 θβ (3.13 Ck m ] y(t m + 1. Ck m m + 1 >, ( 1 σ kmε H σ (t y t (L, t m+1 + εγ [ y t (t y(t m + 1 ]. where γ > is the minimum of coefficients of y t (t 2 2 and y(t. We continue the remaining part as that of [5, Theorem 2.2] to finish the proof. 4. Proof of Theorem 2.3 In this section, we consider the blow-up result in the case of 2m. Set G(t = E 1 E(t + ɛ xy x (ty t (tdx + ρɛ y t (ty(tdx, (4.1 ( β(p with ρ (p 1(1 β, L( 2p, where β is given in the proof of Lemma 3.1 and ɛ is a small and positive constant satisfying G( = E 1 E( + ɛ xy xy 1 dx + ρɛ y 1 y dx >. (4.2 Lemma 4.1. Under the assumptions of Theorem 2.3, we have G(t > for all t. And there exists a positive constant η > such that G (t η[ y t (L, t 2m + y t (L, t 2 + y t (L, t ]. (4.3

6 6 W. LIU, Y. SUN, G. LI EJDE-213/115 Proof. As in [5], using (1.1, (2.1 and Lemma 3.1, we arrive at G (t y t (L, t m+1 + L 2 ɛ y t(l, t 2 + L 2 ɛ y t(l, t 2m + Lɛ y(l, t [ɛ + 2ρɛ](E(t E 1 + 2ρɛ y t (t 2 2 ρɛ y t (L, t m y(l, t + ɛ[ p 1 ρ 2 ] y(t [ɛ + 2ρɛ]E 1 y t (L, t m+1 + L 2 ɛ y t(l, t 2 + L 2 ɛ y t(l, t 2m + + 2ρɛ y t (t 2 2 ρɛ y t (L, t m y(l, t + ɛ [ p 1 ρ 2 β(1 + 2ρ(p 1 ] y(t 2(. Using the choice of ρ and Young s inequality, we obtain Lɛ y(l, t G (t L 2 ɛ y t(l, t 2 + L 2 ɛ y t(l, t 2m + Lɛ y(l, t pρɛ y t(l, t m p ɛρ y(l, t. (4.4 (4.5 Then by repeating similar computations as that of [5, Lemma 4.1], we complete the proof. Set F (t := G 1 α (t + µ where µ is small enough to be chosen later. y t (ty(tdx with α = p 1 2(, (4.6 Proof of Theorem 2.3. (Sketch By repeating similar computations as that of [5, Theorem 2.3], from (3.1 we obtain F (t (1 αg α (t ( η µck 1 1 α [ yt (L, t 2m + y t (L, t 2 + y(l, t ] + 2µ y t 2 2 2µ(E(t E 1 2µE 1 µαk 1 α G 1 α (t + µ p 1 y(t (1 αg α (t ( η µck 1 1 α [ yt (L, t 2m + y t (L, t 2 + y(l, t ] + 2µ y t 2 2 µαk 1 α G 1 α (t + µ(p 1(1 β y(t, where K > to be chosen later. Applying the Cauchy-Schwarz inequality, the Sobolev embedding and Lemma 3.1 to (4.1, we obtain G(t E 1 E(t + Lɛ ( Lɛ 2 + ρɛ + y x (t y t (t dx + ρɛ y t (t β(p 1 y(t 2(, ( Lɛ 2 + ρc2 ɛ y t (t y(t dx y x (t 2 2 (4.7

7 EJDE-213/115 BLOW-UP OF SOLUTIONS 7 where c is the Sobolev embedding constant of y 2 c y x 2. From (2.1 and Lemma 3.1 it follows that y t (t y x (t 2 2 = 2E(t + 2 y(t Combining (4.7 and (4.8, we obtain < 2E y(t 2 + β(p 1 < y(t. (4.8 G(t C y(t. (4.9 Continuing as in the proof of [5, Theorem 2.3] we can complete the proof. Acknowledgments. This work was partly supported by the Qing Lan Project of Jiangsu Province, by grant from the National Natural Science Foundation of China, grant from the the Tianyuan Fund of Mathematics, and grant CXLX12 49 from the JSPS Innovation Program. References [1] M. Aassila, M. M. Cavalcanti, V. N. Domingos Cavalcanti; Existence and uniform decay of the wave equation with nonlinear boundary damping and boundary memory source term, Calc. Var. Partial Differential Equations 15 (22, no. 2, [2] L. Bociu, I. Lasiecka; Blow-up of weak solutions for the semilinear wave equations with nonlinear boundary and interior sources and damping, Appl. Math. (Warsaw 35 (28, no. 3, [3] L. Bociu, I. Lasiecka; Uniqueness of weak solutions for the semilinear wave equations with supercritical boundary/interior sources and damping, Discrete Contin. Dyn. Syst. 22 (28, no. 4, [4] M. M. Cavalcanti, V. N. Domingos Cavalcanti, I. Lasiecka; Well-posedness and optimal decay rates for the wave equation with nonlinear boundary damping source interaction, J. Differential Equations 236 (27, no. 2, [5] H. Feng, S. Li, X. Zhi; Blow-up solutions for a nonlinear wave equation with boundary damping and interior source, Nonlinear Anal. 75 (212, no. 4, [6] V. Georgiev, G. Todorova; Existence of a solution of the wave equation with nonlinear damping and source terms, J. Differential Equations 19 (1994, no. 2, [7] H. A. Levine; Instability and nonexistence of global solutions to nonlinear wave equations of the form P u tt = Au + F (u, Trans. Amer. Math. Soc. 192 (1974, [8] H. A. Levine; Some additional remarks on the nonexistence of global solutions to nonlinear wave equations, SIAM J. Math. Anal. 5 (1974, [9] S. Li, H. Feng and M. Wu; Blow-up solutions for a string equation with nonlinear boundary source and arbitrary-initial-energy, Nonlinear Anal. 75 (212, no. 14, [1] W. J. Liu, J. Yu; On decay and blow-up of the solution for a viscoelastic wave equation with boundary damping and source terms, Nonlinear Anal. 74 (211, no. 6, [11] X. P. Liu; On existence, uniform decay rates, and blow-up for solutions of a nonlinear wave Equation with dissipative and source, Abstr. Appl. Anal. 212 (212, Art , 27 pages. [12] S. A. Messaoudi; Blow up in a nonlinearly damped wave equation, Math. Nachr. 231 (21, [13] F. Tahamtani; Blow-up results for a nonlinear hyperbolic equation with Lewis function, Bound. Value Probl. 29 (29, Art , 9 pp. [14] E. Vitillaro; Global nonexistence theorems for a class of evolution equations with dissipation, Arch. Ration. Mech. Anal. 149 (1999, no. 2, [15] E. Vitillaro; Global existence for the wave equation with nonlinear boundary damping and source terms, J. Differential Equations 186 (22, no. 1,

8 8 W. LIU, Y. SUN, G. LI EJDE-213/115 [16] S. T. Wu; Blow-up of solutions for a system of nonlinear wave equations with nonlinear damping, Electron. J. Differential Equations 29 (29, no. 15, [17] B. Yamna, B. Benyattou; Blow up of solutions for a semilinear hyperbolic equation, E. J. Qualitative Theory of Diff. Equ., 212 (212, no. 4, [18] Y. Ye; Global existence and asymptotic behavior of solutions for some nonlinear hyperbolic equation, J. Inequal. Appl. 21, Art. ID , 1 pp. [19] H. Zhang, Q. Hu; Asymptotic behavior and nonexistence of wave equation with nonlinear boundary condition, Commun. Pure Appl. Anal. 4 (25, no. 4, Wenjun Liu College of Mathematics and Statistics, Nanjing University of Information Science and Technology, Nanjing 2144, China address: wjliu@nuist.edu.cn Yun Sun College of Mathematics and Statistics, Nanjing University of Information Science and Technology, Nanjing 2144, China address: Gang Li College of Mathematics and Statistics, Nanjing University of Information Science and Technology, Nanjing 2144, China address: ligang@nuist.edu.cn