Abstract
We first study how to make use of the Marotto theory to prove rigorously the existence of the Li-Yorke chaos in diffusively coupled map lattices with open boundary conditions (i.e., a high-dimensional discrete dynamical system). Then, the recent 0-1 test for chaos is applied to confirm our theoretical claim. In addition, we control the chaotic motions to a fixed point with delay feedback method. Numerical results support the theoretical analysis.
1. Introduction
Extensive research has been carried out to discover complex behaviors of various discrete dynamical systems in the past several decades. However, limited rigorous analysis concerning existence of chaos in high-dimensional discrete dynamical systems has been seen in the literature. Since the 1980s, coupled map lattices (CMLs) as high-dimensional discrete system have caused widespread concern [1]. CMLs as chaotic dynamical system models for spatiotemporal complexity are usually adopted. Spatiotemporal complexity is common in nature, such as biological systems, networks of DNA, economic activities, and neural networks [1]. The complex behaviors of CMLs have been studied extensively [1–16]. These mainly include bifurcation [2], chaos [6, 7], chaotic synchronization [4, 8–10], and controlling chaos [5, 11, 12]. However, being able to rigorously prove the existence of chaos in CMLs is an important and open question. A rigorous verification of chaos will provide a theoretical foundation for the researchers to discover the complex behaviors in CMLs. Recently, Li et al. [13, 14] theoretically analyzed the chaos in one-way coupled logistic lattice with periodic boundary conditions and presented a chaotification method for creating spatiotemporal systems strongly chaotic. Tian and Chen [15] discussed the chaos in CMLs with the new chaos definition in the sense of Li-Yorke. These CMLs with the periodic boundary conditions have been most extensively investigated [1, 2, 4–15]. But, in all of the research so far published, only a few studies have attempted to explore the case of open boundary conditions [16, 17]. In this case, it is almost impossible to obtain all eigenvalues of Jacobian matrix of the CMLs. This partially hindered early research in the CMLs with open boundary conditions.
Until now, the rigorous proof of chaos has not yet been studied in diffusively coupled map lattices (DCMLs) with open boundary conditions, which is one important case of CMLs. Inspired by the ideas of [13, 14, 18, 19], we have tried to answer this question. The DCML is as follows [1, 16, 17]: where is discrete time step and is lattice point (; is the number of the sites in the DCML). is the coupling strength. represents the state variable for the th site at time . Throughout this paper, we adopt open boundary conditions [16, 17]: Here each of the lattice points in (1.1) and (1.2) is chosen to be the logistic map , where and . The logistic function is equivalent to the well-known form [20] when the transformations and are taken. This simple quadratic iteration was only completely understood in the late 1990s [21]. When the lattice points are logistic functions, the CMLs generate more rich and complex dynamic behaviours. What is more is that the dynamical behaviors of CMLs may be different from each other when the lattice points are chosen from and , respectively [1, 2].
Based on the Marotto theory [22, 23], we prove theoretically the existence of the Li-Yorke chaos in the DCML (1.1). In the process of proving, the most difficult problem is how to find a snap-back repeller. At the same time, we have exploited different measures such as the chaotic phase, bifurcation diagram, and 0-1 test on time series to confirm our claim of the existence of chaos. The 0-1 test is a new method to distinguish chaotic from ordered motion. It is more suitable to handle high-dimensional systems and does not require phase space reconstruction. Finally, we control spatiotemporal chaotic motion in the DCML (1.1) to period-1 orbit (fixed point) by delay feedback and obtain the stability conditions of control.
The paper is organized as follows. In Section 2, the Marotto theorem is introduced. In Section 3.1, a mathematically rigorous proof of the Li-Yorke chaos in the DCML (1.1) is examined. In Section 3.2, we show numerical simulation results. In Section 3.3, 0-1 test method is used to verify the existence of chaos. In Section 4, delay feedback control method is adopted to control chaos. In the last section, conclusions are given.
2. Marotto Theorem
Li and Yorke [24] state that the period-three orbit exhibits chaos in one-dimensional discrete interval map. This is the first precise definition of discrete chaos. This classical criterion for chaos is extended to higher-dimensional discrete systems by Marotto [22]. Marotto considered the following -dimensional discrete system: where and is continuous. Let denote the closed ball in of radius centered at point and its interior. Also, let be the usual Euclidean norm of in [22]. Then, if is differentiable in , Marotto claimed that in the following, .AAll eigenvalues of the Jacobian of system (2.1) at the fixed point are greater than one in norm.BThere exist some and such that, for all .
Marotto thought that, if (A) is satisfied, then (B) can be derived, that is, F is expanding in [22]. But, (A) does not always imply (B) with usual Euclidean norm [25]. Chen et al. [26] first pointed out this problem in the Marotto theorem. During the past decade, several papers tried to fix this error ([19, 23, 25, 26] and some references therein).
In 2005, Marotto redefined the definition of snap-back repeller [23]. He pointed out that (A) does imply (B) with some vector norm in (which depends on and ). See, for example, the discussion by Hirsch and Smale in [27]. However, we still do not know what the vector norm is in specific issues. In the application of the Marotto theorem, we need to find a suitable vector norm. With this special vector norm, (A) implies (B). The correct Marotto theorem is given as follows.
Definition 2.1 (see [23]). Suppose that is a fixed point of (2.1) with all eigenvalues of exceeding 1 in magnitude, and suppose that there exists a point in a repelling neighborhood of , such that and for , where . Then, is called a snap-back repeller of .
Lemma 2.2 (see [23], the Marotto theorem). If has a snap-back repeller, then is chaotic.
At the same time, Shi and Chen [19] presented a modified Marotto theorem as follows.
Lemma 2.3 (see [19]). Consider the n-dimensional discrete system
where is a map from to itself. Assume that has a fixed point satisfying .
Assume, moreover, that(1) is continuously differentiable in a neighborhood of , and all eigenvalues of have absolute values large than 1, where is the Jacobian of F evaluated at , which implies that there exist an and a norm in such that is expanding in , the closed ball of radius centered at in ,(2) is a snap-back repeller of F with , for some and some positive integer , where is the open ball of radius centered at in . Furthermore, is continuously differentiable in some neighborhoods of , respectively, and , where for .
Then, the system (2.2) is chaotic in the sense of Li-Yorke. Moreover, the system (2.2) has positive topological entropy. Here the topological entropy of is defined to be the supremum of topological entropies of restricted to compact invariant sets.
Remark 2.4. The Marotto theorem is a sufficient condition for the Li-Yorke chaos. Lemmas 2.2 and 2.3 have the same effect. But, direct application of the Marotto theorem is not always easy. In most cases, the verification must be carried out with the aid of a computer [28].
3. Proving Chaos and Simulation Verifications
3.1. Proving Chaos
In this subsection, we prove the existence of the Li-Yorke chaos in the DCML (1.1). Lemmas 3.1 and 3.2 will be useful throughout the proof.
Lemma 3.1 (see [29, 30]). For a matrix with eigenvalues , the determinant of A is equal to . Denote .
Lemma 3.2 (see [29, 30], the Gershgorin circle theorem). Let be an matrix, and let denote the circle in the complex plane with center and radius ; that is, where denotes the complex plane. The eigenvalues of are contained within . Moreover, the union of any of these circles that do not intersect the remaining contains precisely (counting multiplicities) of the eigenvalues.
Theorem 3.3. If and is small enough, , and , then the DCML (1.1) is chaotic in the sense of Li-Yorke.
Proof. We will prove that the DCML (1.1) has a snap-back repeller . Rewrite the DCML (1.1) in the vector form as follows:
where and denotes the vector (or matrix) transpose. Using Definition 2.1 and Lemma 2.3, we have to verify the following three conditions.(a) is a fixed point of and all the eigenvalues of have absolute values larger than 1. Moreover, there exist and a norm in such that is expanding in .(b)There exist a and such that for some and .(c).
The proof consists of four steps. The ideas are motivated chiefly by [13, 18, 19].
Step 1. Let , where , . Then is a fixed point of the DCML (3.2), that is, . is continuously differentiable in for some . Its Jacobian matrix at is
where . We denote by , where
Obviously, is not a circulant matrix. When is large, it will be difficult to calculate all the eigenvalues of the matrix . With the Marotto theorem (Lemmas 2.2 and 2.3), we do not need to know the size of eigenvalues and only need to know that the absolute value of eigenvalues is greater than one. According to the Gershgorin circle theorem (Lemma 3.2), all the eigenvalues of , , are given by . Under the conditions of Theorem 3.3, that is, and , the following results are obtained:
that is, all the eigenvalues of are greater (in absolute value) than one. is an expanding fixed point of . Therefore, there exist some and a special vector norm such that is expanding in . That is, for any two distinct points , , we have
where and , are sufficiently close to . Since , where as [19], specially, . When is small enough, we can prove that the operator is expanding with Frobenius matrix norm , where . With the conditions of Theorem 3.3, we get . For any point and small enough, there exists some such that
Since is continuously differentiable, is also expanding for . Let the bound of the maximal open expanding ball be denoted by , where satisfies the following inequality [13]:
Moreover, the equation
has two solutions
where . One has (because is a quadratic function, the discriminant , when , ). In fact, ). Since and , we take . Then, , and we denote
Thus, condition of Definition 2.1 and Lemma 2.2 is satisfied.Step 2. For all , we have , that is, , where . Now let and , that is,
where . Summing all the above equations, we obtain
Assume that (3.12) has a solution, and denote , that is , which has two solutions: . We choose since and .Step 3. Now, let , that is,
where . Summing the above equations, we get
Assume that the system of (3.14) has a solution, and denote , that is, , that is,, which has two solutions: . We take . Thus,
Denote ; since and , then
Denoting , we get . So, is monotone increasing continuous function, and . We get (since the condition ). Therefore, . On the other hand, . Thus, , and . Let , ; then, . Steps 2 and 3 complete the proof of condition (b).Step 4. According to , where , with Lemma 3.2, all eigenvalues of lie in the interval . Thus, with Lemma 3.1, . Moreover, according to , where , with Lemma 3.2, all eigenvalues of lie in the interval . Thus, with Lemma 3.1, . Then, we have and . Thus, condition (c) is complete. The system (1.1) has a snap-back repeller . Under the conditions of the Theorem 3.3, the DCML (1.1) is chaotic in the sense of Li-Yorke. The proof is completed.
3.2. Numerical Simulation of Chaos
When , , and , the conditions of Theorem 3.3 are satisfied. The DCML (1.1) can be denoted as follows: The corresponding eigenvalues of lie in the interval , that is, . These eigenvalues are strictly larger than one in absolute value. Starting from a random initial state, the number of iterations is 140. Simulation result is shown in Figure 1. When fixed , , and ; these satisfy the conditions of Theorem 3.3. Thus, the system (3.18) should display chaotic behavior. The bifurcation diagram in Figure 2 also confirms the above statement.
3.3. 0-1 Test for Chaos in the DCML
The 0-1 test for chaos was first reported in [31]. It and its modified version are applied directly to the time series data and do not require phase space reconstruction [31–36]. Moreover, the dimension and origin of the dynamical system are irrelevant. The 0-1 test can efficiently distinguish chaotic behavior from regular (periodic or quasiperiodic) behavior in deterministic systems. The test result is 0 or 1, depending on whether the dynamics is regular or chaotic, respectively. This method has been successfully applied to some typical chaotic systems [37–44] and experiment data [45]. We apply this method to the DCML. From another point of view, we show the existence of chaos in the DCML using the 0-1 test. Now, we describe the implementation of the 0-1 test. The interested reader can consult [35] for further details. Consider discrete data sets sampled at times , where is the total number of data points. is an observable data from the underlying dynamic system.
Step 1. For a random number , define the translation variables
Step 2. Define the mean square displacement as follows: Note that this definition requires . In practice, yields good results. Denote , where the function rounds the elements of to the nearest integers.
Step 3. Define the modified mean square displacement where and .
Step 4. Form the vectors and . Then define the correlation coefficient
Step 5. Steps 1–4 are performed for values of chosen randomly in the interval . In practice, is sufficient. We then compute the median of these values of to compute the final result . indicates regular dynamics, and indicates chaotic dynamics.
Note that the -trajectories provide a direct visual test of whether the underlying dynamics is chaotic or nonchaotic. Namely, bounded trajectories in the -plane imply regular dynamics, whereas Brownian-like (unbounded) trajectories imply chaotic dynamics [31, 32]. With the sufficient length of the time series, indicates that the dynamics is regular and indicates that the dynamics is chaotic [43].
Now, we apply the 0-1 test to the DCML (3.18). Fix , and choose a random initial point ; we carry out the 0-1 test with and , respectively. Using the data set of in the system (3.18), we get at and at . The translation variables are shown in Figures 3 and 4, respectively.
We take , and let vary from 0.01 to 0.058 in increments of 0.01. It is clear that the computed value of is effective for most values of in Figure 5. These 0-1 test results are consistent with numerical simulation in Section 3.2 and Theorem 3.3 in Section 3.1. Here we stress that the test results (chaos or nonchaos) are independent of the choices of initial point and changing the observable does not greatly alter the computed value of .
4. Control Spatiotemporal Chaos
When , , and , the system (3.18) displays chaotic dynamics. The DCML (3.18) has an unstable equilibrium point . The goal of this section is to control spatiotemporal chaotic motions in the DCML (3.18) to the equilibrium point using delay feedback [46, 47]. We rewrite the DCML (3.18) as where .
Theorem 4.1. With the local controllers , the chaotic motion in the DCML (4.1) (i.e., (3.18)) can be controlled to the fixed point , where .
Proof. Since the local controllers are given by we get the controlled DCML: where . Expanding (4.3) around the fixed point , we obtain Since , and , we have . Thus, we get and Then, by using (4.4) and (4.5), we get For the sake of simplicity, we denote by ; then where , , and . With the Gershgorin circle theorem (Lemma 3.2), we get that is, Solving inequality (4.10), we obtain . Since , we get . The proof is completed.
The simulation result is shown in Figure 6. Chaotic motions are quickly controlled to the fixed point .
Remark 4.2. In the process of proving Theorems 3.3 and 4.1, we only need to know that eigenvalues are greater (or less) than one in absolute and it is not necessary to compute explicitly the eigenvalues. These ideas avoid difficulties in calculating eigenvalues in higher-dimension DCMLs using the Gershgorin circle theorem.
5. Conclusion
With the Marotto theorem and the Gershgorin circle theorem, we have theoretically analyzed the chaos in the DCML with open boundary conditions, which presents a theoretical foundation for chaos analysis of the DCML. What is more is that the 0-1 test further confirms the existence of chaos and we control spatiotemporal chaotic motions in the DCML to period-1 orbits. Stability analysis is presented. The results of simulations are consistent with theoretical analysis. We wish to emphasize that the methods of this paper can be used in all those cases where the eigenvalues of Jacobi matrix are difficult to calculate in CMLs.
Acknowledgment
The paper was supported by NSFC (no. 10871074).