LASALLE TYPE STATIONARY OSCILLATION THEOREMS for AFFINE-PERIODIC SYSTEMS Hongren Wang Xue Yang Yong Li Xiaoyue Li
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DISCRETE AND CONTINUOUS doi:10.3934/dcdsb.2017156 DYNAMICAL SYSTEMS SERIES B Volume 22, Number 7, September 2017 pp. 2907{2921 LASALLE TYPE STATIONARY OSCILLATION THEOREMS FOR AFFINE-PERIODIC SYSTEMS Hongren Wang College of mathematics, Jilin Normal University Jilin 136000, China College of Mathematics, Jilin University Changchun 130012, China Xue Yang School of Mathematics and Statistics, Northeast Normal University Changchun 130024, China Center for Mathematics and Interdisciplinary Sciences, Northeast Normal University Changchun 130024, China College of Mathematics, Jilin University Changchun 130012, China Institute of Mathematics, Academy of Mathematics and Systems Science Chinese Academy of Sciences, Beijing 100190, China Yong Li∗ School of Mathematics and Statistics, Northeast Normal University Changchun 130024, China Center for Mathematics and Interdisciplinary Sciences, Northeast Normal University Changchun 130024, China College of Mathematics, Jilin University Changchun 130012, China Xiaoyue Li School of Mathematics and Statistics, Northeast Normal University Changchun 130024, China (Communicated by Yuan Lou) Abstract. The paper concerns the existence of affine-periodic solutions for affine-periodic (functional) differential systems, which is a new type of quasi- periodic solutions if they are bounded. Some more general criteria than LaSalle's one on the existence of periodic solutions are established. Some applications are also given. 2010 Mathematics Subject Classification. Primary: 34C27; Secondary: 34C25. Key words and phrases. LaSalle type stationary oscillation principle, Q-affine T -periodic solu- tions, asymptotic stability. The second author is supported by NSFC (grant No. 11201173). The third author is supported by National Basic Research Program of China (grant No. 2013CB834100), NSFC (grant No. 11571065) and NSFC (grant No. 11171132). ∗ Corresponding author: Yong Li. 2907 2908 HONGREN WANG, XUE YANG, YONG LI AND XIAOYUE LI 1. Introduction. As is well known, regular solutions of differential equations are generally split into the following types: stationary, periodic, quasi-periodic, almost periodic, almost automorphic or recurrent ones. It is an important topic what conditions, for example, stability, can guarantee the existence of such solutions. Some classical results of this aspect can be found in books [27] and [1]. Here we would like to mention LaSalle's stationary oscillation theorem [13], which is a simple but flexible criterion in applications. More precisely, consider the equation d x_ = f(t; x)(· ≡ ); (1) dt where f : R1 ×Rn ! Rn is continuous, T -periodic in t, and satisfies some regularity conditions. 1 1 Theorem 1.1. (LaSalle stationary oscillation principle, [13]) Let a : R+ ! R+ nf0g be continuous and satisfy lim a(t) = 0. Assume f(t; x) is Lipschitz in x, T -periodic t!1 1 in t, and all solutions are bounded on R+; moreover jx(t; x0) − x(t; y0)j ≤ a(t)jx0 − y0j 8t ≥ 0; where x(t; x0) denotes the solution of system (1) with the initial value x(0) = x0. Then system (1) has a unique asymptotically stable T -periodic solution. Some systems exhibit the following affine periodicity: −1 1 n f(t + T; x) = Qf(t; Q x) 8t 2 R ; x 2 R ; where Q is a nonsingular n × n matrix. When Q = I (the identity matrix), system (1) is just a usual T -periodic one; when Q is an orthogonal matrix, system (1) is a quasi-periodic one. From physics' views it is natural to ask whether system (1) admits a solution x(t) with the following property x(t + T ) = Qx(t) 8t; where f(t; x) possesses the affine periodicity. We call such a solution x(t) a Q-affine T -periodic solution of (1). As mentioned above, a Q-affine T -periodic solution x(t) might be T -periodic if Q = I, quasi- periodic if Q is orthogonal, even unbounded like eλty(t) with quasi-periodic y(t) if Q is only nonsingular. For periodic solutions and anti-periodic solutions, some works can be found in [2, 4, 5, 7, 8, 10, 11, 12, 15, 16, 17, 18, 19, 21, 22, 25, 26]. Recently, there have been some literature in studying the existence of affine-periodic solutions, see, for example, [3, 14, 20, 23, 24, 28]. In this paper, we will prove similar stationary oscillation theorems for Q-affine periodic systems with orthogonal matrix Q, make some discusses about more general stationary oscillation conditions, and give more flexible versions. The paper is organized as follows. In Section 2, we state a stationary oscillation theorem on Q-affine T periodic solutions. Section 3 provides an analogous result for functional differential equations. In Section 4, we illustrate some applications. 2. Stationary oscillation theorem for ODE (1). From now on we always as- sume Q is an orthogonal matrix. In this section, we will prove the following LaSalle type theorem on Q-affine T periodic solutions for system (1). Theorem 2.1. Consider the Q-affine T -periodic system (1), and assume f : R1 × n n 1 1 R ! R is continuous. Let a : R+ ! R+ n f0g be continuous and satisfy r = LASALLE TYPE STATIONARY OSCILLATION THEOREMS 2909 lim a(kT ) < 1. Assume the following hold: k!1 1) There exists a solution z(t) of system (1) defined on [0;T ]; 2) Any two solutions x(t) and y(t) satisfy jx(t) − y(t)j ≤ a(t)jx(0) − y(0)j whenever they exist. Then system (1) has a unique asymptotically stable Q-affine T periodic solution. Lemma 2.2. Assume the Q-affine T -periodic system (1) admits the uniqueness to initial value problems. Let x(t; x0) be a solution of (1) with the initial value 1 condition x(0) = x0 defined on R+. Then −1 −1 Q x(t + T; x0) ≡ x(t; Q x(T; x0)): 1 Proof. Note that for t 2 R+, Z t+T x(t + T; x0) = x0 + f(s; x(s; x0))ds 0 Z T Z t+T = x0 + f(s; x(s; x0))ds + f(s; x(s; x0))ds 0 T Z t = x(T; x0) + f(s + T; x(s + T; x0))ds 0 Z t −1 = x(T; x0) + Q f(s; Q x(s + T; x0))ds; 0 which together with the uniqueness of solutions to the initial value problem implies that −1 −1 Q x(t + T; x0) ≡ x(t; Q x(T; x0)): We now give the proof of Theorem 2.1. Proof. We take the following three steps: 1 Step 1. Prove that z(t) is extensible on R+. Once done, by 2), it follows that jx(t; x0) − z(t)j ≤ a(t)jx0 − z(0)j 8t ≥ 0: 1 Hence all the solutions x(t; x0) of system (1) are defined on R+. By 2), x(t; x0) is uniquely determined by the initial value x0, i.e., the equation admits the continuous dependence of solutions with respect to initial values. By Peano's theorem and 2), (1) has a unique solution w(t) with the initial value condition w(0) = Q−1z(T ), and it exists on [0;T ]. Note that dQw(t − T ) dQw(t − T ) = dt d(t − T ) = Qf(t − T;Q−1Qw(t − T )) = f(t; Qw(t − T )) 8t 2 [T; 2T ]: Therefore, Qw(t − T ) is the solution of (1) with the initial value condition z(T ) on [T; 2T ]. 2910 HONGREN WANG, XUE YANG, YONG LI AND XIAOYUE LI Define 8 < z(t); t 2 [0;T ]; z(t) = : Qw(t − T ); t 2 [T; 2T ]: Hence z(t) exists on [T; 2T ]. Repeating this procedure, we obtain that z(t) exists 1 on R+. Step 2. Prove that the equation has Q-affine T periodic solutions. We define the Poincar´emap P : Rn ! Rn by −1 n P (x0) = Q x(T; x0) 8x0 2 R ; and P is well-defined. By the definition of r, there exist positive integer K and r 2 (r; 1) such that a(kT ) ≤ r 8k ≥ K: By induction and the uniqueness of solutions to initial value problem, we have j j−1 −j P (x0) = P ◦ P (x0) = Q x(jT; x0) 8j ≥ 1: In fact, j j−1 −1 j−1 P (x0) = P ◦ P (x0) = Q x(T;P x0) 8j ≥ 1: Note that Z t+T x(t + T; x0) = x0 + f(s; x(s; x0))ds 0 Z t+T = x(T; x0) + f(s; x(s; x0))ds: T Therefore Z t −1 −1 −1 Q x(t + T; x0) = Q x(T; x0) + f(s; Q x(s + T; x0))ds: 0 Further we have Z t −1 −1 −1 x(t; Q x(T; x0)) = Q x(T; x0) + f(s; x(s; Q x(T; x0)))ds: 0 From the above, we can see −2 −1 −1 Q x(t + T; x0) = Q x(t; Q x(T; x0)): In the same manner, it follows that j −j P (x0) = Q x(jT; x0) 8j ≥ 1: Thus K K −K −K jP (x0) − P (y0)j = jQ x(KT; x0) − Q x(KT; y0)j = jx(KT; x0) − x(KT; y0)j n ≤ rjx0 − y0j 8x0; y0 2 R : Therefore P is a r-contraction mapping of order K. Now, by Banach's contraction n mapping fixed point theorem, P has a unique fixed point x∗ 2 R , i.e., −1 Q x(T; x∗) = x∗: Again by the uniqueness of solutions to the initial value problem, we have x(t + T; x∗) ≡ Qx(t; x∗) 8t; LASALLE TYPE STATIONARY OSCILLATION THEOREMS 2911 which shows that x(t; x∗) is a unique Q-affine T periodic solution of the system. Step 3. Prove the stability and the asymptotic stability. Let us prove the following estimate inductively: m jx(t + mKT; x∗) − x(t + mKT; x0)j ≤ a(t)jx∗ − x0jr 8t ≥ 0; m ≥ 1: Firstly, for m = 1, by Lemma 1 and assumption 2), we have jx(t + KT; x∗) − x(t + KT; x0)j −1 −1 =jQ x(t + KT; x∗) − Q x(t + KT; x0)j −1 −1 =jx(t + (K − 1)T;Q x(T; x∗)) − x(t + (K − 1)T;Q x(T; x0))j −1 −1 −1 −1 =jQ x(t + (K − 1)T;Q x(T; x∗)) − Q x(t + (K − 1)T;Q x(T; x0))j −1 −1 =jx(t + (K − 2)T;Q x(T;Q x(T; x∗))) −1 −1 − x(t + (K − 2)T;Q x(T;Q x(T; x0)))j −2 −2 =jx(t + (K − 2)T;Q x(2T; x∗)) − x(t + (K − 2)T;Q x(2T; x0))j = :::::: −K −K =jx(t; Q x(KT; x∗)) − x(t; Q x(KT; x0))j ≤a(t)jx(KT; x∗) − x(KT; x0)j ≤a(t)a(KT )jx∗ − x0j ≤a(t)jx∗ − x0jr: Secondly, suppose that for k = m − 1, the estimate holds.