# B5.6 Nonlinear Systems

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1 B5.6 Nonlinear Systems 5. Global Bifurcations, Homoclinic chaos, Melnikov s method Alain Goriely 2018 Mathematical Institute, University of Oxford

2 Table of contents 1. Motivation 1.1 The problem 1.2 A paradigm 1.3 Step 1: Bernoulli Shift 1.4 Step 2: Smale s horseshoe 1

3 What we learned from Section 1&2. For a system of linear autonomous equations ẋ = Ax, the solutions live on invariant spaces that can be classified according to the eigenvalues of A. The stable (resp. unstable, centre) linear subspace is the span of eigenvectors whose eigenvalues have a negative (resp. positive, null) real part. For nonlinear systems, we define the notion of asymptotic sets (α and ω limit set), the notion of attracting set, and basin of attraction. We define two important notions of stability for a fixed point: (Lyapunov) stability (i.e. solutions remain close ) and exponential stability i.e. ( fixed point is stable AND all nearby solutions converge to the fixed point asymptotically for long time ). Lyapunov functions can be used to test stability. But, finding a Lyapunov function can be difficult. 2

4 What we learned from Section 3. For a system of autonomous equations ẋ = f(x), we are interested in the trajectories and asymptotic sets in phase space. At a fixed point, we can define local stable, unstable, and centre manifolds based on the corresponding linear subspaces of the linearised system. From the local stable and unstable manifolds, we can define the global stable and unstable manifolds by extending them to all times. If the unstable manifold is non-empty, the fixed point is unstable. If the unstable and centre manifolds are empty, the fixed point is asymptotically stable. If the unstable manifold is empty but the centre manifold is non-empty, we can study the dynamics on the centre manifold by centre manifold reduction. The same notions can be defined for iterative mappings and for periodic orbits. 3

5 What we learned from Section 4. For a system ẋ = f(x, µ), we are interested in bifurcations. Bifurcations are parameter values where a qualitative change occurs. For local bifurcations (at fixed points), it implies the existence of eigenvalues of the Jacobian matrix crossing the imaginary axis. At the bifurcation value, there is a centre manifold. Hence, we can use centre manifold techniques. Adding the parameter as a variable, we can define an extended centre manifold and use results of Section 3. We find generic behaviours of bifurcations changing the type or number of fixed points (saddle-node, transcritical, pitchfork). The Hopf bifurcation leads to the possibility of transforming a fixed point into a limit cycle. The same notions apply to mappings and periodic orbits. Bifurcation of maps show the new possibility of period-doubling (leading to chaos). 4

6 4. Motivation

7 4. Motivation 4.1 The problem

8 The problem Consider the first-order system of differential equations ẋ = f(x) + εg(x, t) where x : E R n, (1) and assume that g is periodic in t ( T > 0, s.t. g(x, t + T ) = g(x, t).) Assuming, we know the dynamics of the system when ε = 0 and that it supports periodic and homoclinic orbits. Problem: What happens when ε > 0? Are there still periodic orbits? Homoclinic orbits? New orbits? 5

9 4. Motivation 4.2 A paradigm

10 An important example: the Duffing oscillator For ε = 0. ẍ = x x 3 + ε (δẋ + γ cos(t)) (2) y x

11 An important example: the Duffing oscillator For ε > 0, γ = 0, δ > 0. ẍ = x x 3 + ε (δẋ + γ cos(t)) (3) What happens when ε > 0, γ > 0, δ > 0? = CHAOS! For what values? What does chaos mean? 7

12 Main idea Our construction will be in four steps of increasing complexity Step 1: Bernoulli shift (the simplest dynamical system with chaos) Step 2: Smale s horseshoe (a geometric construction) Step 3: Homoclinic chaos in ODEs Step 4: Melnikov s method (an explicit method to detect chaos) 8

13 4. Motivation 4.3 Step 1: Bernoulli Shift

14 A simple dynamical system To define a dynamical system we need: - A phase space Σ. - The dynamics on Σ (how elements of Σ are mapped to other elements). 1. The phase space. For the Bernoulli shift we define Σ as the set of bi-infinite sequence of 0 and 1: s Σ : s = {..., s n,..., s 1 s 0, s 1,..., s n,...}, (4) where s i is equal to 0 or 1. 9

15 Bernoulli Shift s Σ : s = {..., s n,..., s 1 s 0, s 1,..., s n,...}, (5) where s i is equal to 0 or 1. Notion of distance on Σ. Take two elements s, s Σ: d(s, s ) = i Z Tow elements are close if their central blocks agree, s i s i 2 i (6) 10

16 Bernoulli Shift 2. The dynamics on Σ Define the shift map σ : Σ Σ. If s = {..., s n,..., s 2, s 1 s 0, s 1,..., s n,...}, (7) then Equivalently σ(s) = {..., s n,..., s 1, s 0, s 1,..., s n,...}, (8) (σ(s)) i = s i+1. (9) Possible orbits? 11

17 Bernoulli shift Theorem 4.1 The shift map has: 1. a countable infinity of periodic orbits with arbitrary periods; 2. an uncountable infinity of non-periodic orbits; 3. a dense orbit. What is a dense orbit? Definition 4.2 A dense orbit for the shift map is a particular orbit s d Σ such that for any s Σ and ɛ > 0, n N such that d(σ n (s d ), s) < ɛ. 12

18 Bernoulli shift Theorem 4.3 The shift map has: 1. a countable infinity of periodic orbits with arbitrary periods; 2. an uncountable infinity of non-periodic orbits; 3. a dense orbit. Proof: 13

19 Sensitive dependence to initial conditions Two important notions in dynamical systems. Let Λ be an invariant compact set for an invertible iterative map f : M M. Definition 4.4 f has sensitivity to initial conditions on Λ if ɛ > 0 such that for any p Λ and any neighbourhood U of p, there exists p U and n N such that f n (p) f n (p ) > ɛ. Definition 4.5 f is topologically transitive on Λ if for any open sets U, V Λ n Z such that f n (U) V. 14

20 Sensitive dependence to initial conditions Together they lead to the notion of chaos: Definition 4.6 Let Λ be an invariant compact set for an invertible iterative map f : M M. Then f is chaotic on Λ if it has sensitivity to initial conditions on Λ and is topologically transitive on Λ. Theorem 4.7 The shift map is chaotic on Σ. 15

21 4. Motivation 4.4 Step 2: Smale s horseshoe

22 Smale s horseshoe The construction of Smale s horseshoe is given in the file B56-Section5b.pdf 16

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