Topological Conjugacy for Lipschitz Perturbations of Non-Autonomous Systems
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DISCRETE AND CONTINUOUS doi:10.3934/dcds.2016017 DYNAMICAL SYSTEMS Volume 36, Number 9, September 2016 pp. 5011{5024 TOPOLOGICAL CONJUGACY FOR LIPSCHITZ PERTURBATIONS OF NON-AUTONOMOUS SYSTEMS Ming-Chia Li∗ and Ming-Jiea Lyu Department of Applied Mathematics National Chiao Tung University, 1001 University Road Hsinchu 300, Taiwan (Communicated by Lan Wen) Abstract. In this paper, topological conjugacy for two-sided non-hyperbolic and non-autonomous discrete dynamical systems is studied. It is shown that if the system has covering relations with weak Lyapunov condition determined by a transition matrix, there exists a sequence of compact invariant sets restricted to which the system is topologically conjugate to the two-sided subshift of finite type induced by the transition matrix. Moreover, if the systems have covering relations with exponential dichotomy and small Lipschitz perturbations, then there is a constructive verification proof of the weak Lyapunov condition, and so topological dynamics of these systems are fully understood by symbolic representations. In addition, the tolerance of Lipschitz perturbation can be characterised by the dichotomy tuple . Here, the weak Lyapunov condition is adapted from [12, 24, 15] and the exponential dichotomy is from [2]. 1. Introduction. In the study of dynamical systems, getting topological conju- gacy to a symbolic system is a popular method to realize the long-term qualitative behavior of a system from topological viewpoints. Under uniformly hyperbolic con- dition, the classical results are presented by Smale [21] for horseshoe maps, Hirsch and Pugh [8] for hyperbolic maps, and Robinson [19] for structurally stable maps, etc. Without hyperbolicity, there are computable methods to obtain topological semi-conjugacy; for instance, Zgliczy´nski[22, 23] introduced covering relations, Mis- chaikov et al. [4,5, 13] used discrete Conley index, Yorke et al. [10,9] presented expanders family, and Richeson et al. [7, 17] for index systems. Furthermore to obtain topological conjugacy, in our previous work [15, 16], we brought in the Lya- punov condition, which is adapted from the cone condition in [12, 24]. In this paper, we weaken the Lyapunov condition and establish a method to verify it, but still secure the truth of the conjugacy result. Non-autonomous discrete systems are time-dependent so that the maps describ- ing the evolution are allowed to change with time; while autonomous systems are time-independent and can be viewed as a particular case of non-autonomous ones. 2010 Mathematics Subject Classification. 37B55, 37D25, 37C15, 37B10. Key words and phrases. Topological conjugacy, exponential dichotomy, Lipschitz perturbation, non-autonomous systems, nonuniformly hyperbolic systems. This work was partially supported by MOST grant 104-2115-M-009-003-MY2. The authors are grateful to the anonymous referee for valuable suggestions. ∗ Corresponding author: M.-C. Li. 5011 5012 MING-CHIA LI AND MING-JIEA LYU Such systems arise quite naturally in practical applications, for example, the Eu- ler method with a variable time-step hn > 0 for a differential equationx _ = f(x) gives a non-autonomous system of the form xn+1 = xn + hnf(xn): In recent years there has been growing interest in non-autonomous problems; refer to [1,6] for surveys with open problems and [11, 18] for monographs in this topics. For non- uniformly hyperbolic dynamics, Barreira and Valls [2] established a version of the Grobman-Hartman theorem by constructing topological conjugacy between a lin- ear non-autonomous system with non-uniform exponential dichotomy and its small Lipschitz perturbations. Moreover, in [3], they discussed the relation between non- uniform exponential dichotomy and strict Lyapunov sequence. For motivational purpose, let us illustrate interesting dynamics with the following simple example. For each integer n, let Dn be a constant such that 1 ≤ Dn ≤ 3 if n ≥ 0 and 2 ≤ Dn ≤ 3 if n < 0; and define a map fn on the real line by ( Dnx − 1 + Ln; for x 2 R and Dn ≥ 2; fn(x) = 2 2 (1) ±Dnx ± Ln x + sin (2πx) ; for x Q 0 and Dn < 2; where Ln > 0 is a small constant. Then (fn)n2Z forms a two-sided non-autonomous system with linear part (Dn)n2Z and Lipschitz perturbation involving (Ln)n2Z.A bounded orbit of the system is a two-sided bounded sequence (xn)n2Z of numbers satisfying xn+1 = fn(xn) for all integers n. It is clear that if Dn ≥ 2 for all n, then P1 Qi −1 xn ≡ −1 + i=n(Di − Li) j=n Dj is the unique bounded orbit of system (1) and it persists under small Lipschitz perturbations. If Dn = 1 for all n ≥ 0, then Pn Qi −1 xn ≡ i=−1(1 − Li) j=n Dj for n < 0 and xn ≡ 0 for n ≥ 0 form a bounded orbit for the system; nevertheless, the uniqueness of bounded orbits requires the assumption that the one-sided sequence (Ln)n≥0 is not summable, and hence it dose not necessarily persist under perturbations. For topological conjugacy from system (1) to its small perturbations, it gives rise to the question whether a unique bounded orbit persists under perturbations. It is not difficult to see that the answer is yes if the limit inferior of Dn is not equal to one as n goes to infinity, while the system is uniformly hyperbolic in some sense. However, the answer is no if the Q1 limit of Dn is equal to one as n goes to infinity, the infinite product n=0 Dn of the one-sided sequence (Dn)n≥0 is convergent, and the one-sided sequence (Ln)n≥0 is summable, while the system is neither uniformly hyperbolic nor exponentially dichotomic. This leads us to consider the general case when the system is not uniformly hyperbolic but is exponentially dichotomic. Notice that if one of Dn's is less than 2, then we cannot apply the result of [15, 16], since a Lyapunov condition of quadratic form is not satisfied. In this paper, instead of just a unique bounded orbit in the above example, we consider the general case, namely, bounded invariants sets for high-dimensional systems, possibly with both stable and unstable directions. We first use covering relations of Zgliczy´nski[22, 23] to change local coordinate along stable and unstable directions, then impose weak Lyapunov condition in form of a sequence of real- valued functions to track displacements of two orbits, and finally obtain topological conjugacy. More precisely, for a two-sided non-autonomous system having covering relations with weak Lyapunov condition determined by a transition matrix A, we show that the system has a sequence of compact invariant sets restricted to which the system is topologically-conjugate to σA, the two-sided subshift of finite type induced by A. TOPOLOGICAL CONJUGACY FOR NON-AUTONOMOUS SYSTEMS 5013 In addition, we show that if the system has linear part admitting exponential dichotomy of Barreira and Valls [2] and non-linear part with small Lipschitz pertur- bation, then the covering relations must admit the weak Lyapunov condition and hence a subsystem has dynamics exactly as subshift dynamics. Moreover, we give the tolerance of Lipschitz perturbations controlled by the dichotomy tuple. 2. Statements of definitions and main results. First, we introduce some basic notations and definitions. For a positive integer K, let RK denote the space of all K-tuples of real numbers, |·| be the Euclidean norm on RK , and let k · k denote the operator-norm on the space of linear transformations on RK induced by |·|. For x 2 RK and r > 0, we denote BK (x; r) = fz 2 RK : jz − xj < rg; for the particular case when x = 0 and r = 1, we write BK = BK (0; 1), that is, the open unit ball in RK . Moreover, for a subset M of RK , let M, int(M) and @M denote the closure, interior and boundary of S, respectively. A function from a set into itself is also called a map on the set. Let N denote the set of all non-negative integers, while Z denotes the set of all integers. Let R+ denote the non-negative real numbers. We briefly recall some definitions from [25] concerning covering relations. Definition 2.1 (See [25, Definition 6]). An h-set in RK is a quadruple consisting of the following data: • a non-empty compact subset M of RK ; • a pair of numbers u(M); s(M) 2 f0; 1; :::; kg with u(M) + s(M) = K; and K K u(M) s(M) u(M) • a homeomorphism cM : R ! R = R × R with cM (M) = B × Bs(M): For simplicity, we will denote such an h-set by M, and say that the h-set M has unstable (respectively, stable) direction, if u 6= 0 (respectively, s 6= 0); furthermore, we use the following notations: M~ = Bu(M) × Bs(M); M~ − = @Bu(M) × Bs(M); M~ + = Bu(M) × @Bs(M); − −1 ~ − + −1 ~ + the unstable boundary M = cM (M ); and the stable boundary M = cM (M ); we also writev ~ = cM (v) for v 2 M. A covering relation between two h-sets is defined as follows. Definition 2.2 (See[25, Definition 7]). Let M; N be h-sets in RK with u(M) = u(N) = u and s(M) = s(N) = s; f : M ! Ru × Rs be a continuous function, and ~ −1 ~ u s f := cN ◦ f ◦ cM : M ! R × R . We say M f-covers N, denoted by f M =) N; if the following conditions are satisfied: 1. there exists a homotopy h : [0; 1]×M~ ! Ru ×Rs such that h([0; 1]; M~ −)\N~ = ;, h([0; 1]; M~ ) \ N~ + = ;, and h(0; x~) = f~(~x) for allx ~ 2 M~ ; 2. there exists a function : Ru ! Ru such that (@Bu) ⊂ RunBu and h(1; p; q) = ( (p); 0) for all p 2 Bu and q 2 Bs; and 3.