On local aspects of sensitivity in topological dynamics
 Lei Liu^{1}Email author,
 Haitao Li^{2} and
 Jinli Pang^{3}
https://doi.org/10.1186/s1366201506055
© Liu et al. 2015
Received: 26 January 2015
Accepted: 11 August 2015
Published: 9 September 2015
Abstract
In this paper along with the research on weakly mixing sets and transitive sets, we introduce a local aspect of sensitivity in topological dynamics and give the concept of an sset. It is shown that a weakly mixing set is an sset. A transitive set with the set of periodic points being dense is an sset. In particular, a transitive set is an sset for interval maps. Moreover, we discuss ssets for setvalued discrete dynamical systems.
Keywords
MSC
1 Introduction
A topological dynamical system (abbreviated by TDS) is a pair \((X,f)\), where X is a compact metric space with metric d and \(f:X\to X\) is a continuous map. When X is finite, it is a discrete space and there is no nontrivial convergence at all. Hence, we assume that X contains infinitely many points. Let \(\mathbb{N^{+}}\) denote the set of all positive integers and let \(\mathbb{N}=\mathbb{N^{+}}\cup \{0\}\).
Transitivity, weak mixing, and sensitive dependence on initial conditions (see [1–4]) are global characteristics of topological dynamical systems. Let \((X,f)\) be a TDS, \((X,f)\) is (topologically) transitive if for any nonempty open subsets U and V of X there exists an \(n\in\mathbb{N}\) such that \(f^{n}(U)\cap V\neq\emptyset\). \((X,f)\) is (topologically) mixing if for any nonempty open subsets U and V of X, there exists an \(N\in\mathbb{N}\) such that \(f^{n}(U)\cap V\neq\emptyset\) for all \(n\in\mathbb{N}\) with \(n\geq N\). \((X,f)\) is (topologically) weakly mixing if for any nonempty open subsets \(U_{1}\), \(U_{2}\), \(V_{1}\), and \(V_{2}\) of X, there exists an \(n\in\mathbb{N}\) such that \(f^{n}(U_{1})\cap V_{1}\neq\emptyset\) and \(f^{n}(U_{2})\cap V_{2}\neq\emptyset\). It follows from these definitions that mixing implies weak mixing, which in turn implies transitivity. A map f is said to have sensitive dependence on initial conditions if there is a constant \(\delta>0\) such that for any nonempty open set U of X, there exist points \(x,y\in U\) such that \(d(f^{n}(x),f^{n}(y))> \delta\) for \(n\in\mathbb{N^{+}}\).
In [5], Blanchard introduced overall properties and partial properties. For example, sensitive dependence on initial conditions, Devaney chaos (see [6]), weak mixing, mixing and more belong to overall properties; LiYorke chaos (see [7]) and positive entropy (see [1, 8]) belong to partial properties. Weak mixing is an overall property, it is stable under semiconjugate maps and implies LiYorke chaos. We find that a weakly mixing system always contains a dense uncountable scrambled set (see [9]). In [10], Blanchard and Huang introduced the concepts of a weakly mixing set, derived from a result given by Xiong and Yang [11] and showed ‘partial weak mixing implies LiYorke chaos’ and ‘LiYorke chaos cannot imply partial weak mixing’.
Motivated by the idea of Blanchard and Huang’s notion of a ‘weakly mixing subset’, Oprocha and Zhang [12] extended the notion of a weakly mixing set and gave the concept of a transitive set and discussed its basic properties. In this paper we give the concept of ‘sset’ for topological dynamical systems and investigate the relationship among transitive subsets, weakly mixing sets, and ssets. We find that a TDS to have a weakly mixing set implies it has an sset, and if periodic points are dense in the transitive set, then the transitive set is an sset. In particular, a transitive set is an sset for interval maps. The properties of transitivity, weak mixing, and sensitivity on initial conditions for a setvalued discrete dynamical system were discussed (see [13–18]). Also, we continue to discuss ssets for setvalued discrete dynamical systems and investigate the relationship between a setvalued discrete system and an original system on an sset. More precisely, a setvalued discrete system has an sset, which implies that an original system has an sset.
2 Preliminaries
A TDS \((X,f)\) is point transitive if there exists a point \(x_{0}\in X\) with dense orbit i.e., \(\overline{\operatorname{orb}(x_{0})}=X\). Such a point \(x_{0}\) is called a transitive point of \((X,f)\). In general, transitive and point transitive are independent (see [2]). A TDS \((X,f)\) is minimal if \(\overline{\operatorname{orb}(x,f)}=X\) for every \(x\in X\), i.e., every point is transitive point. A point x is called minimal if the subsystem \((\overline{\operatorname{orb}(x,f)},f)\) is minimal. Denote by \(P(f)\) the set of all periodic points.
 (1)
\(\lim _{n\to\infty}\inf d(f^{n}(x),f^{n}(y))=0\), and
 (2)
\(\lim _{n\to\infty}\sup d(f^{n}(x),f^{n}(y))>0\).
 (1)
f is transitive,
 (2)
f is periodically dense; i.e., the set of periodic points of f is dense in X, and
 (3)
f is sensitive dependent on initial conditions.
Definition 2.1
[10]
Let \((X,f)\) be a TDS and A be a closed subset of X with at least two elements. A is said to be weakly mixing if for any \(k\in\mathbb{N}\), any choice of nonempty open subsets \(V_{1},V_{2},\ldots,V_{k}\) of A and nonempty open subsets \(U_{1},U_{2},\ldots,U_{k}\) of X with \(A\cap U_{i}\neq\emptyset\), \(i=1,2,\ldots,k\), there exists an \(m\in\mathbb{N}\) such that \(f^{m}(V_{i})\cap U_{i}\neq\emptyset\) for \(1\leq i\leq k\). \((X,f)\) is called partial weak mixing if X contains a weakly mixing subset.
Definition 2.2
[12]
Let \((X,f)\) be a TDS and A be a nonempty subset of X. A is called a transitive set of \((X,f)\) if, for any choice of nonempty open subset \(V^{A}\) of A and nonempty open subset U of X with \(A\cap U\neq\emptyset\), there exists an \(n\in\mathbb{N}\) such that \(f^{n}(V^{A})\cap U\neq\emptyset\).
Remark 2.1
 (1)
\((X,f)\) is topologically transitive if and only if X is a transitive set of \((X,f)\).
 (2)
By [12], A is a transitive set if and only if A̅ is a transitive set, where A̅ denotes the closure of A.
 Result 1.:

If A is a weakly mixing set of \((X,f)\), then A is a transitive set of \((X,f)\).
 Result 2.:

If \(a\in X\) is a transitive point of \((X,f)\), then \(\{a\}\) is a transitive set of \((X,f)\).
 Result 3.:

If \(A=\operatorname{orb}(x,f)\) is a periodic orbit of \((X,f)\) for some \(x\in X\), then A is a transitive set of \((X,f)\).
Definition 2.3
A nonempty subset A is called an sset of \((X,f)\) if there exists a \(\delta>0\) such that for any \(x\in A\) and \(\varepsilon>0\), there exist a \(y\in B(x,\varepsilon)\cap A\) and an \(n\in\mathbb{N^{+}}\) satisfying \(d(f^{n}(x),f^{n}(y))>\delta\).
Remark 2.2
The sset is dense in itself, i.e., it contains no isolated points. X is an sset of \((X,f)\) if and only if \((X,f)\) is sensitive dependent on initial conditions.
Let \((X,f)\) and \((Y,g)\) be two TDSs. Then \((X,f)\) is an extension of \((Y,g)\), or \((Y,g)\) is a factor of \((X,f)\) if there exists a surjective continuous map \(h: X\to Y\) (called a factor map) such that \(h\circ f(x)=g\circ h(x)\) for every \(x\in X\). If further h is a homeomorphism, then \((X,f)\) and \((Y,g)\) are said to be topologically conjugate and the homeomorphism h is called a conjugated map.
Theorem 2.1
Let A be a weakly mixing set of \((X,f)\). Then A is an sset of \((X,f)\).
Proof
Let A be a weakly mixing set of \((X,f)\). Then A contains at least two points. Pick up two distinct points \(x_{1},x_{2}\in A\) and set \(\delta=\frac{1}{4}d(x_{1},x_{2})>0\). For any \(x\in A\) and \(\varepsilon>0\), we see that \(B(x,\varepsilon)\) is a nonempty open subset of X.
3 Characterizing ssets
Lemma 3.1
Let A be an infinite subset of X and \(P(f)\) be dense in A. Then \(P(f)\cap A\) is infinite.
Proof
Suppose that \(P(f)\cap A\) is finite and let \(\operatorname{card}(P(f)\cap A)=n\), where card represents the cardinality of a set. Since A is an infinite subset of X, there exists a pairwise disjoint open set \(V_{i}^{A}\) of A for \(i=1,2,\ldots,n+1\), i.e., \(V_{i}^{A}\cap V_{j}^{A}=\emptyset\) for \(i,j\in\{1,2,\ldots ,n+1\}\) and \(i\neq j\). Moreover, \(P(f)\cap A\) is dense in A, which implies \(\operatorname{card}(P(f)\cap A)\geq n+1\). This is a contradiction. Therefore, \(P(f)\cap A\) is infinite. □
Theorem 3.1
Let \((X,f)\) be a TDS and A be an infinite subset of X. If A is a transitive set of \((X,f)\) and \(P(f)\) is dense in A, then A is an sset of \((X,f)\).
Proof
By [19], if X is a nondegenerate compact interval, \(f:X\to X\) is a continuous map and f is transitive, then \(P(f)\) is dense in X. We prove that if X is a nondegenerate compact interval, A is a nondegenerate closed interval, and A is a transitive set of \((X,f)\), then \(P(f)\) is dense in A. □
Lemma 3.2
[19]
Suppose that I is a nondegenerate interval and \(f:I\to I\) is a continuous map. If \(J\subseteq I\) is an interval which contains no periodic points of f and z, \(f^{m}(z)\) and \(f^{n}(z)\in J\) with \(0< m< n\), then either \(z< f^{m}(z)< f^{n}(z)\) or \(z>f^{m}(z)>f^{n}(z)\).
Theorem 3.2
Let I be a nondegenerate interval and \(f:I\to I\) be a continuous map. If \(J\subseteq I\) is a nondegenerate closed interval and J is a transitive set of \((I,f)\), then \(P(f)\) is dense in J.
Proof
Suppose that \(P(f)\) is not dense in J. Then there exists a nonempty open set \(J_{1}\) of J containing no periodic points, i.e., \(P(f)\cap J_{1}=\emptyset\). Without loss of generality, let \(J_{1}\) be an open interval of I and \(J_{1}\subseteq J\). Take an \(x\in J_{1}\) which is not an endpoint of \(J_{1}\), an open neighborhood \(U\subsetneq J_{1}\) of x and an open interval \(E\subseteq J_{1}\setminus U\).
We consider open neighborhood U of \(J_{1}\) and open interval E with \(J_{1}\cap E\neq\emptyset\). Since J is a transitive set of \((I,f)\), there exists an \(m\in\mathbb{N}\) such that \(f^{m}(U)\cap E\neq\emptyset\). Furthermore, there exists a \(y\in U\subseteq J_{1}\) such that \(f^{m}(y)\in E\subseteq J_{1}\). Moreover, \(P(f)\cap J_{1}=\emptyset\), it means that \(y\neq f^{m}(y)\). Since I is a Hausdorff space and f is continuous, there exists an open neighborhood V of y such that \(f^{m}(V)\cap V=\emptyset\). Hence, we can take an open interval \(J_{2}\) of I such that \(J_{2}\subseteq J_{1}\) and \(y\in J_{2}\subseteq V\), thus \(f^{m}(J_{2})\cap J_{2}=\emptyset\). Since \(J_{2}\) is an open interval of I and \(J_{2}\subseteq J_{1}\) and J is a transitive set, there exist \(n>m\) and \(z\in U\) such that \(f^{n}(z)\in J_{2}\). Furthermore, we have \(0< m< n\) and \(z, f^{n}(z)\in J_{2}\) while \(f^{m}(z)\notin J_{2}\). This is a contradiction by Lemma 3.2. Therefore, \(P(f)\) is dense in J. □
Example 3.1
4 ssets for setvalued discrete dynamical systems
Let X be a \(T_{1}\) space, that is, a single point set that is closed. Then \(\kappa(A)=\{F\in\kappa(X): F\subseteq A\}\) is a closed subset of \(\kappa(X)\) for any nonempty closed subset A of X (see [20]).
Theorem 4.1
Let A be a nonempty closed subset of X. If \(P(f)\) is dense in A, then \(P(\bar{f})\) is dense in \(\kappa(A)\).
Proof
Let \({\mathcal{V}}^{\kappa(A)}\) be a nonempty open subset of \(\kappa(A)\). Then there exists an open set \({\mathcal{V}}\) of \(\kappa (X)\) such that \({\mathcal{V}}^{\kappa(A)}={\mathcal{V}}\cap\kappa(A)\). Without loss of generality, let \({\mathcal{V}}=\nu(V_{1},V_{2},\ldots,V_{m})\). Take \(F\in{\mathcal{V}}\cap\kappa(A)\). Then we have \(F\subseteq A\), \(F\subseteq\bigcup _{i=1}^{m} V_{i}\) and \(F\cap V_{i}\neq\emptyset\) for each \(i=1,2,\ldots,m\). Hence, \(V_{i}\cap A\neq\emptyset\) for each \(i=1,2,\ldots,m\). Since \(P(f)\) is dense in A, it follows that \(P(f)\cap(V_{i}\cap A)\neq\emptyset\) for each \(i=1,2,\ldots,m\). Furthermore, there exist \(y_{i}\in P(f)\cap(V_{i}\cap A)\) and \(n_{i}\in\mathbb{N^{+}}\) such that \(f^{n_{i}}(y_{i})=y_{i}\) for each \(i=1,2,\ldots,m\). Let \(G=\{y_{1},y_{2},\ldots,y_{m}\}\). Then \(G\in {\mathcal{V}}\) and \(G\in\kappa(A)\), which implies \(G\in {\mathcal{V}}^{\kappa(A)}\). Moreover, \(f^{n_{1}n_{2}\cdots n_{m}}(y_{i})=y_{i}\) for each \(i=1,2,\ldots,m\). Therefore, \((\bar{f})^{n_{1}n_{2}\cdots n_{m}}(G)=f^{n_{1}n_{2}\cdots n_{m}}(G)=G\), it means that \(P(\bar{f})\cap {\mathcal{V}}^{\kappa(A)}\neq\emptyset\). This shows that \(P(\bar{f})\) is dense in \(\kappa(A)\). □
Theorem 4.2
Let A be a nonempty closed subset of X. If \(\kappa(A)\) is a sensitive set of \((\kappa(X),\bar{f})\), then A is an sset of \((X,f)\).
Proof
Since \(\kappa(A)\) is an sset of \((\kappa(X),\bar{f})\), there exists a constant \(\delta>0\) such that \(K\in\kappa(A)\) and every \(\varepsilon>0\) there exist \(G\in B(K,\varepsilon)\cap\kappa(A)\) and \(n\in\mathbb{N^{+}}\) such that \(d_{H}((\bar{f})^{n}(K),(\bar{f})^{n}(G))>\delta\).
Declarations
Acknowledgements
The work was supported by the National Natural Science Foundation of China (11401363) and the Education Foundation of Henan Province (13A110832, 14B110006), P.R. China. The authors would like to thank the referees for many valuable and constructive comments and suggestions for improving this paper.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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