Hindawi Publishing Corporation Abstract and Applied Analysis Volume 2011, Article ID 526026, 13 pages doi:10.1155/2011/526026 Research Article Multiplicity of Solutions for Nonlocal Elliptic System of p, q-Kirchhoff Type Bitao Cheng,1 Xian Wu,2 and Jun Liu1 1 College of Mathematics and Information Science, Qujing Normal University, Qujing, Yunnan 655011, China 2 Department of Mathematics, Yunnan Normal University, Kunming, Yunnan 650092, China Correspondence should be addressed to Bitao Cheng, chengbitao2006@126.com Received 16 May 2011; Accepted 23 June 2011 Academic Editor: Josip E. Pečarić Copyright q 2011 Bitao Cheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper is concerned with the following nonlocal elliptic system of p,q-Kirchhoff type p−1 q−1 −M1 Ω |∇u|p Δp u λFu x, u, v, in Ω, −M2 Ω |∇v|q Δq v λFv x, u, v, in Ω, u v 0, on ∂Ω. Under bounded condition on M and some novel and periodic condition on F, some new results of the existence of two solutions and three solutions of the above mentioned nonlocal elliptic system are obtained by means of Bonanno’s multiple critical points theorems without the PalaisSmale condition and Ricceri’s three critical points theorem, respectively. 1. Introduction and Preliminaries We are concerned with the following nonlocal elliptic system of p, q-Kirchhoff type: p−1 − M1 Δp u λFu x, u, v, |∇u|p in Ω, q−1 Δq v λFv x, u, v, − M2 |∇v|q in Ω, Ω Ω u v 0; 1.1 on ∂Ω, where Ω ⊂ RN N ≥ 1 is a bounded smooth domain, λ ∈ 0, ∞, p > N, q > N, Δp is the p-Laplacian operator Δp u div |∇u|p−2 ∇u , and Mi : R → R, i 1, 2, are continuous functions with bounded conditions. 1.2 2 Abstract and Applied Analysis M There are two positive constants m0 , m1 such that m0 ≤ Mi t ≤ m1 , ∀t ≥ 0, i 1, 2. 1.3 Furthermore, F : Ω × R × R → R is a function such that Fx, s, t is measurable in x for all s, t ∈ R × R and Fx, s, t is C1 in s, t for a.e. x ∈ Ω, and Fu denotes the partial derivative of F with respect to u. Moreover, Fx, s, t satisfies the following. F1 Fx, 0, 0 0 for a.e. x ∈ Ω. F2 There exist two positive constants γ < p, β < q and a positive real function αx ∈ L∞ Ω such that |Fx, s, t| ≤ αx 1 |s|γ |t|β , for a.e. x ∈ Ω and all s, t ∈ R × R. 1.4 The system 1.1 is related to a model given by the equation of elastic strings ∂2 u ρ 2 − ∂t E P0 h 2L L 2 2 ∂u dx ∂ u 0 ∂x ∂x2 1.5 0 which was proposed by Kirchhoff 1 as an extension of the classical D’Alembert’s wave equation for free vibrations of elastic strings, where the parameters in 1.5 have the following meanings: ρ is the mass density, P0 is the initial tension, h is the area of the cross-section, E is the Young modulus of the material, and L is the length of the string. Kirchhoff’s model takes into account the changes in length of the string produced by transverse vibrations. Later, 1.5 was developed to the form utt − M Ω |∇u|2 Δu fx, u in Ω, 1.6 where M : R → R is a given function. After that, many people studied the nonlocal elliptic boundary value problem −M Ω |∇u|2 Δu fx, u in Ω, u 0 on ∂Ω, 1.7 which is the stationary counterpart of 1.6. It is pointed out in 2 that 1.7 models several physical and biological systems, where u describes a process which depends on the average of itself e.g., population density. By using the methods of sub and supersolutions, variational methods, and other techniques, many results of 1.7 were obtained, we can refer to 2–12 and the references therein. In particular, Alves et al. 2, Theorem 4 supposes that M satisfies bounded condition M and fx, t satisfies condition AR, that is, for some ν > 2 and R > 0 such that 0 < ν Fx, t ≤ fx, tt, ∀|t| ≥ R, x ∈ Ω, AR Abstract and Applied Analysis 3 t where Fx, t 0 fx, sds; one positive solutions for 1.7 was obtained. It is well known that condition AR plays an important role for showing the boundedness of Palais-Smale sequences. More recently, Corrêa and Nascimento in 13 studied a nonlocal elliptic system of p-Kirchhoff type p−1 Δp u fu, v ρ1 x, in Ω, − M1 |∇u|p Ω p−1 Δp v gu, v ρ2 x, in Ω, − M2 |∇v|p P Ω ∂u ∂v 0, on ∂Ω, ∂η ∂η where η is the unit exterior vector on ∂Ω, and Mi , ρi i 1, 2, f, g satisfy suitable assumptions, where a special and important condition: periodic condition on nonlinearity was assumed. They obtained the existence of a weak solution for the nonlocal elliptic system of p-Kirchhoff type P under Neumann boundary condition via Ekeland’s Variational Principle. In the present paper, our objective is to consider the nonlocal elliptic system of p, q-Kirchhoff-type 1.1, instead of the nonlocal elliptic system of p-Kirchhoff type and single Kirchhoff type equation. Under bounded condition on M and some novel conditions without P S condition and periodic condition on F, we will prove the existence of two solutions and three solutions of system 1.1 by means of one multiple critical points theorem without the Palais-Smale condition of Bonanno in 14 and an equivalent formulation 15, Theorem 2.3 of Ricceri’s three critical points theorem 16, Theorem 1, respectively. In order to state our main results, we need the following preliminaries. 1,p 1,q Let X W0 Ω × W0 Ω be the Cartesian product of two Sobolev spaces, which is a reflexive real Banach space endowed with the norm u, v u p v q , 1,p 1.8 1,q where · p and · q denote the norms of W0 Ω and W0 Ω, respectively. That is, u p 1,p Ω |∇u|p 1/p v q , Ω |∇v|q 1/q 1.9 1,q for all u ∈ W0 Ω and v ∈ W0 Ω. 1,p 1,q Since p > N and q > N, W0 Ω and W0 Ω are compactly embedded in C0 Ω. Let C max ⎧ ⎨ ⎩ sup 1,p u∈W0 Ω\{0} maxx∈Ω |ux|p p u p , sup 1,q v∈W0 Ω\{0} ⎫ maxx∈Ω |vx|q ⎬ ; q ⎭ v q 1.10 4 Abstract and Applied Analysis then we have C < ∞. Furthermore, it is known from 17 that 1/N maxx∈Ω |ux|p p − 1 1−1/P 1/N−1/P N N −1/P sup Γ 1 ≤ √ , |Ω| 2 p−N u p π 1,p u∈W Ω\{0} 0 1.11 where Γ denotes the Gamma function and |Ω| is the Lebesgue measure of Ω. Additionally, 1.11 is an equality when Ω is a ball. Recall that u, v ∈ X is called a weak solution of system 1.1 if M1 Ω |∇u|p p−1 Ω −λ Ω q−1 |∇u|p−2 ∇u∇ϕ M2 |∇v|q |∇v|q−2 ∇v∇ψ Ω Fu x, u, vϕxdx − λ Ω Ω 1.12 Fx, u, vdx 1.13 Fv x, u, vψxdx 0, for all ϕ, ψ ∈ X. Define the functional I : X → R given by 1 Iu, v M 1 p Ω |∇u| p 1 M 2 q Ω |∇v| q −λ Ω for all u, v ∈ X, and where 1 t M t M1 sp−1 ds, 0 2 t M t M2 sq−1 ds, ∀t ≥ 0. 1.14 0 By the conditions M and F2, it is easy to see that I ∈ C1 X, R and a critical point of I corresponds to a weak solution of the system 1.1. Now, giving x0 ∈ Ω and choosing R2 > R1 > 0 such that Bx0 , R2 ⊆ Ω, where Bx, R N {y ∈ R : |y − x| < R}. Next we give some notations. 1/P N 1/P C1/P RN π N/2 2 − R1 , R2 − R1 Γ1 N/2 1/q N 1/q C1/q RN π N/2 2 − R1 . R2 − R1 Γ1 N/2 α1 α1 N, p, R1 , R2 α2 α2 N, q, R1 , R2 1.15 Abstract and Applied Analysis 5 Moreover, let a, c be positive constants, denote ⎛ ⎞ N 2 1/2 a ⎝ ⎠, R2 − yx xi − x0i R2 − R1 i1 Ac s, t ∈ R × R : |s|p |t|q ≤ c , ka Bx0 ,R2 \Bx0 ,R1 hc, a ka − gc, ∀x ∈ Bx0 , R2 \ Bx0 , R1 , gc F x, yx, yx dx ⎧ ⎫ ⎨ mp−1 mq−1 ⎬ 1 , 1 , M max ⎩ p q ⎭ sup Fx, s, tdx, Ω s,t∈Ac Bx0 ,R1 Fx, a, adx, ⎧ ⎫ ⎨ mp−1 mq−1 ⎬ 0 M− min , 0 . ⎩ p q ⎭ 1.16 Now we are ready to state our main results for the system 1.1 Theorem 1.1. Assume that F1-F2 hold and there are three positive constants a, c1 , c2 with c1 < aα1 p aα2 q ≤ 1 < c2 such that M gc1 < M− hc1 , a, M gc2 < M− hc1 , a. 1.17 Then, for each λ∈ ! " M aα1 p aα2 q M− c1 aα1 p aα2 q 1 1 , , min , Chc1 , a C gc1 gc2 1.18 there exists a positive real number ρ such that the system 1.1 has at least two weak solutions ui , vi ∈ X i 1, 2 whose norms in C0 Ω are less than some positive constant ρ. Theorem 1.2. Assume that F1-F2 hold and there are two positive constants a, b, with aα1 p aα2 q > bM /M− such that F3 Fx, s, t ≥ 0 for a.e. x ∈ Ω \ Bx0 , R1 and all s, t ∈ 0,a × 0, a; F4 aα1 p aα2 q |Ω|supx,s,t∈Ω×AbM /M− Fx, s, t < b Bx0 ,R1 Fx, a, adx. Then there exist an open interval Λ ⊆ 0, ∞ and a positive real number ρ such that, for each λ ∈ Λ, the system 1.1 has at least three weak solutions wi ui , vi ∈ X i 1, 2, 3 whose norms wi are less than ρ. 2. Proofs of Main Results Before proving the results, we state one multiple critical points theorem without the PalaisSmale condition of Bonanno in 13 and an equivalent formulation 14, Theorem 2.3 of Ricceri’s three critical points theorem 15, Theorem 1, which are our main tools. 6 Abstract and Applied Analysis Theorem 2.1 see 14, Theorem 2.1. Let X be a reflexive real Banach space, and let Ψ, Φ : X → R be two sequentially weakly lower semicontinuous functions. Assume that Ψ is (strongly) continuous and satisfies lim u → ∞ Ψu ∞. Assume also that there exist two constants r1 and r2 such that j infX Ψ < r1 < r2 ; jj ϕ1 r1 < ϕ2 r1 , r2 ; jjj ϕ1 r2 < ϕ2 r1 , r2 ; where ϕ1 r Φu − infu∈Ψ−1 −∞,rw Φu inf r − Ψu u∈Ψ−1 −∞,r ϕ2 r1 , r2 inf u∈Ψ−1 −∞,r1 , 2.1 Φu − Φv . sup Ψv − Ψu v∈Ψ−1 r1 ,r2 Then, for each λ∈ ! " 1 1 1 , min , , ϕ2 r1 , r2 ϕ1 r1 ϕ2 r2 2.2 the functional Ψ λΦ has two local minima which lie in Ψ−1 −∞, r1 and Ψ−1 r1 , r2 , respectively. Theorem 2.2 see 15, Theorem 2.3. Let X be a separable and reflexive real Banach space. Ψ : X → R is a continuously Gâteaux differentiable and sequentially weakly lower semicontinuous functional whose Gâteaux derivative admits a continuous inverse on X ∗ ; Φ : X → R is a continuously Gâteaux differentiable functional whose Gâteaux derivative is compact. Suppose that i lim u → ∞ Ψu λΦu ∞ for each λ > 0; ii There are a real number r, and u0 , u1 ∈ X such that Ψu0 < r < Ψu1 ; iii infu∈Ψ−1 −∞,r Φu > Ψu1 − rΦu0 r − Ψu0 Φu1 /Ψu1 − Ψu0 . Then there exist an open interval Λ ⊆ 0, ∞ and a positive real number ρ such that, for each λ ∈ Λ, the equation Ψ u λΦ u 0 has at least three weak solutions whose norms in X are less than ρ. First, we give one basic lemma. Lemma 2.3. Assume that M and F2 hold; let Ψu, v 1 M1 p Ω |∇u|p 1 M 2 q Ω |∇v|q , Φu, v − Ω Fx, u, vdx, 2.3 for all u, v ∈ X. Then Ψ and Φ are continuously Gâteaux differentiable and sequentially weakly lower semicontinuous functionals. Moreover, the Gâteaux derivative of Ψ admits a continuous inverse on X ∗ and the Gâteaux derivative of Φ is compact. Abstract and Applied Analysis 7 Proof. By condition M, it is easy to see that Ψ is continuously Gâteaux differentiable. Moreover, the Gâteaux derivative of Ψ admits a continuous inverse on X ∗ . Thanks to p > N, q > N, and F2, Φ is continuously Gâteaux differentiable and sequentially weakly lower semicontinuous functional whose Gâteaux derivative is compact. Next We will prove that Ψ is a sequentially weakly lower semicontinuous functional. Indeed, for any un , vn ∈ X with 1,p 1,q un , vn u, v in X, then un u in W0 Ω and vn v in W0 Ω. Therefore, lim inf un p ≥ u p , lim inf vn q ≥ v q n→∞ 2.4 n→∞ due to the weakly lower semicontinuity of norm. Hence by virtue of the continuity and 1 and M 2 , we conclude that monotonicity of M 1 1 lim inf 1 M |∇u|p ≤ M |∇un |p ≤ lim inf M |∇un |p , n→∞ n→∞ Ω Ω Ω q q 2 lim inf 2 2 ≤M M |∇v| |∇vn | ≤ lim inf M |∇vn |q , n→∞ Ω n→∞ Ω 2.5 Ω Consequently, Ψ is a sequentially weakly lower semicontinuous functional. Proof of Theorem 1.1. Let 1 Ψu, v M 1 p Ω |∇u| p 1 M 2 q q Ω |∇v| , Φu, v − Ω Fx, u, vdx 2.6 for all u, v ∈ X. Under condition M, by a simple computation, we have p q p q M− u p v q ≤ Ψu, v ≤ M u p v q . 2.7 Therefore, 2.7 implies that Ψu, v ∞. lim 2.8 u,v → ∞ Put r1 M− c 1 aα1 p aα2 q , C r2 M− c 2 aα1 p aα2 q . C 2.9 Denote ϕ1 r ϕ2 r1 , r2 u,v∈ inf Φu, v − infu,v∈ Ψ−1 −∞,r inf u,v∈ Ψ−1 −∞,r1 Ψ−1 −∞,rw Φu, v , r − Ψu, v Φu, v − Φu1 , v1 , sup u1 ,v1 ∈ Ψ−1 r1 ,r2 Ψu1 , v1 − Ψu, v and Ψ−1 −∞, rw is the closure of Ψ−1 −∞, r in the weak topology. 2.10 8 Abstract and Applied Analysis Set w0 x ⎧ ⎪ 0, ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎛ a ⎝ R2 − ⎪ ⎪ R2 − R1 ⎪ ⎪ ⎪ ⎪ ⎩ a, x ∈ Ω \ Bx0 , R2 , 1/2 ⎞ ⎠, xi − xi N x ∈ Bx0 , R2 \ Bx0 , R1 , 0 2.11 i1 x ∈ Bx0 , R1 . Then u0 , v0 ∈ X, where u0 x v0 x w0 x and p p u0 p w0 p aα1 p , C q q v0 q w0 q aα1 q . C 2.12 Consequently, 2.7 and 2.12 imply that r1 < Ψu0 , v0 < r2 . 2.13 Furthermore, 2.13 implies that ϕ2 r1 , r2 inf sup Φu, v − Φu1 , v1 − Ψu, v u,v∈ Ψ−1 −∞,r1 u ,v ∈ Ψ−1 r ,r Ψu1 , v1 1 1 1 2 Φu, v − Φu0 , v0 . ≥ inf −1 u,v∈Ψ −∞,r1 Ψu0 , v0 − Ψu, v 2.14 On the other hand, by F1, 1.17, and, 2.11, one has M Fx, u0 , v0 dx ka > hc1 , a > gc1 > gc1 M− Ω sup Fx, s, tdx. Ω s,t∈Ac1 2.15 For each u, v ∈ X with Ψu, v ≤ r1 , and x ∈ Ω, by 2.7, we conclude Cr p q 1 c1 aα1 p aα2 q ≤ c1 . |ux|p |vx|q ≤ C u p v q ≤ M− 2.16 Abstract and Applied Analysis 9 Therefore, the combination of 2.15 and 2.16 implies Φu, v − Φu0 , v0 Ψu0 , v0 − Ψu, v ≥ ≥ Fx, u0 , v0 dx − Ω Fx, u, vdx Ψu0 , v0 − Ψu, v Fx, u0 , v0 dx − Ω sup|ux|p |vx|q ≤c1 Fx, u, vdx Ω Ω Ψu0 , v0 − Ψu, v Fx, u0 , v0 dx − Ω sup|ux|p |vx|q ≤c1 Fx, u, vdx Ω Ω ≥ Fx, u0 , v0 dx − M Ψu0 , v0 2.17 sup|ux|p |vx|q ≤c1 Fx, u, v dx p q u0 p v0 q Ω C hc1 , a. M aα1 p aα2 q By 2.14 and 2.17, we have ϕ2 r1 , r2 ≥ M C hc1 , a. aα1 p aα2 q 2.18 Similarly, for every u, v ∈ X such that Ψu, v ≤ r, where r is a positive real number, and x ∈ Ω, one has Cr p q . |ux|p |vx|q ≤ C u p v q ≤ M− 2.19 By virtue of Ψ being sequentially weakly lower semicontinuous, then Ψ−1 −∞, rw Ψ−1 −∞, r. Consequently, ϕ1 r ≤ ≤ inf Φ0, 0 − infu,v∈ Ψ−1 −∞,rw Ψ−1 −∞,rw Φu, v r − Ψ0, 0 −infu,v∈ Ω Φu, v r − Ψu, v u,v∈ Ψ−1 −∞,r ≤ Φu, v − infu,v∈ Ψ−1 −∞,rw 2.20 Φu, v r sup|ux|p |vx|q ≤Cr/M− Fx, u, vdx r . 10 Abstract and Applied Analysis It implies that ϕ1 r1 ≤ gc1 C C hc1 , a, p q gc1 < r1 M− c1 aα1 aα2 M aα1 p aα2 q 2.21 ϕ1 r2 ≤ gc2 C C hc1 , a. p q gc2 < r2 M− c2 aα1 aα2 M aα1 p aα2 q 2.22 By 2.18–2.22, we conclude ϕ1 r1 ≤ ϕ2 r1 , r2 , ϕ1 r2 ≤ ϕ2 r1 , r2 . 2.23 Therefore, the conditions j, jj, and jjj in Theorem 2.1 are satisfied. Consequently, by Lemma 2.3 and above facts, the functional Ψ λΦ has two local minima u1 , v1 , u2 , v2 ∈ X, which lie in Ψ−1 −∞, r1 and Ψ−1 r1 , r2 , respectively. Since I Ψ λΦ ∈ C1 , u1 , v1 , u2 , v2 ∈ X are the solutions of the equation Ψ u, v λΦ u, v 0. 2.24 Then u1 , v1 , u2 , v2 ∈ X are the weak solutions of system 1.1. Since Ψui , vi < r2 , i 1, 2, by 1.10 and 2.7, |ui x|p |vi x|q ≤ Cr2 ≤ c2 , M− 2.25 i 1, 2; which implies there exists a positive real number ρ such that the norms of ui , vi ∈ X i 1, 2 in C0 Ω are less than some positive constant ρ. This completes the proof of Theorem 1.1. Proof of Theorem 1.2. Let Ψu, v 1 M1 p Ω |∇u|p 1 M 2 q Ω |∇v|q , Φu, v − Ω Fx, u, vdx 2.26 for all u, v ∈ X. By F2 and 2.7, we have 1 q Fx, u, vdx |∇u| M2 |∇v| − λ q Ω Ω Ω p q αx 1 |ux|γ |vx|β dx ≥ M− u p v q − λ 1 Ψu, v λΦu, v M 1 p p Ω p q γ β ≥ M− u p v q − λ α ∞ |Ω| k1 u p k2 v q , 2.27 Abstract and Applied Analysis 11 where k1 , k1 are positive constants. Since γ < p, β < q, 2.27 implies that lim Ψu, v λΦu, v ∞. 2.28 u,v → ∞ The same as in 2.11, defining a function w0 x, and letting u0 x v0 x w0 x, then 2.12 is also satisfied. Choosing r bM /C, by 2.7, 2.12, and aα1 p aα2 q > bM /M− , we conclude M M− bM p q − Ψu0 , v0 ≥ M− u0 p v0 q r. aα1 p aα2 q > C C M− 2.29 By F3 and the definitions of u0 and v0 , one has |Ω| sup x,s,t∈Ω×AbM /M− b Fx, a, adx aα1 p aα2 q Bx0 ,R1 Fx, a, adx bM Bx0 ,R1 C M aα1 p aα2 q /C bM Ω\Bx0 ,R1 Fx, u0 , v0 dx Bx0 ,R1 Fx, u0 , v0 dx ≤ p q C M u v Fx, s, t < 0 p bM ≤ C Ω 0 q Fx, u0 , v0 dx . Ψu0 , v0 2.30 For every u, v ∈ X such that Ψu, v ≤ r, and x ∈ Ω, one has Cr C bM bM p q . |ux|p |vx|q ≤ C u p v q ≤ M− M− C M− By the combination of 2.30 and 2.31, we have sup u,v∈Ψ−1 −∞,r −Φu, v ≤ sup {u,v|Ψu,v≤r} sup Ω Fx, u, vdx {u,v||ux|p |vx|q ≤bM /M− } Ω Fx, u, vdx 2.31 12 Abstract and Applied Analysis ≤ sup Ω s,t∈AbM /M− ≤ |Ω| sup Fx, s, tdx x,s,t∈Ω×AbM /M− bM ≤ C r Ω Fx, s, t Fx, u0 , v0 dx Ψu0 , v0 −Φu0 , v0 . Ψu0 , v0 2.32 Therefore, Φu0 , v0 . Ψu0 , v0 2.33 Ψu0 , v0 − rΦ0, 0 r − Ψ0, 0Φu0 , v0 . 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