# 3 Stability and Lyapunov Functions

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1 CDS140a Nonlinear Systems: Local Theory 02/01/ Stability and Lyapunov Functions 3.1 Lyapunov Stability Denition: An equilibrium point x 0 of (1) is stable if for all ɛ > 0, there exists a δ > 0 such that for all x N δ (x 0 ) and t 0, we have φ t (x) N ɛ (x 0 ). An equilibrium point x 0 of (1) is unstable if it is not stable. An equilibrium point x 0 of (1) is asymptotically stable if it is stable and if there exists a δ > 0 such that for all x N δ (x 0 ) we have lim t φ t (x) = x 0. Remarks: The about limit being satised does not imply that x 0 is stable (why?). From H-G theorem and Stable manifold theorem, it follows that hyperbolic equilibrium points are either asymptotically stable (sinks) or unstable (sources or saddles). If x 0 is stable then no eigenvalue of Df(x 0 ) has positive real part (why?) x 0 is stable but not asymptotically stable, then x 0 is a non-hyperbolic equilibrium point Example: Perko (c) Determine stability of the equilibrium points of : [ ] [ ] ẋ1 4x1 2x = ẋ 2 x 1 x 2 Equilibrium points are (0, 2), (1, 0). Df(x) = Df(0, 2) = Df(1, 0) = [ ] 4 2 x 2 x 1 [ ] [ ] What can we say in general about the stability of non-hyperbolic equilibrium points? [ ] [ ] ẋ1 x2 x = 1 x 2 ẋ 2 x 1 + x Lyapunov Functions Denition: Let f C 1 (E), V C 1 (E) and φ t the ow of the dierential equation 1. Then for x E the derivative of the function V (x) along the solution φ t (x) is V (x) = d dt V (φ t(x)) = V (φ t) φ t d dt φ t(x) = DV (x)f(x) Theorem (Lyapunov's Direct Method): Let E be an open subset of R n containing x 0. Suppose f C 1 (E) and that f(x 0 ) = 0. Suppose further that there exists a real valued function V C 1 (E) satisfying 9

2 V (x 0 ) = 0 and V (x) > 0 if x x 0. Then (a) if V (x) 0 for all x E, x 0 is stable; (b) if V (x) < 0 for all x E\{x 0 }, x 0 is asymptotically stable; (c) if V (x) > 0 for all x E\{x 0 }, x 0 is unstable; Proof: Without loss of generality, we assume that x 0 = 0. Part (a) We want to show that for all ɛ > 0, there exists a δ > 0 such that for all x N δ (0) and t 0, we have φ t (x) N ɛ (0) Outline: 1. Construct a closed set (ball) B r N ɛ, such that B r E (i.e., a technicality to make sure we remain in the domain) E B r N 2. Construct Ω β = {x B r V (x) β} (i.e., a subset of the β sublevel set of V ) such that Ω β lies in the interior of B r Can show that condition (a) implies that x Ω β φ t (x) Ω β Figure 1: Sets used in the proof. 3. Construct N δ (0) Ω β Then since N δ (0) Ω β B r N ɛ (0), we have that x N δ Ω β φ t (x) Ω β φ t (x) N ɛ (0). Details: 1. Given any ɛ > 0, choose 0 < r ɛ, such that B r = {x R n x r} E. 2. Let α = min x =r V (x) (i.e., the minimum of V in the boundary of B r ). Take 0 < β < α, Ω β = {x B r V (x) β}. Then it can be easily shown that Ω β lies in the interior of B r (if a point a is in the boundary, then V (a) α > β). Notice that for x = φ 0 (x) Ω β, and for all t V (φ t (x)) V (φ 0 (x)) = ˆ t V (φ t (x)) V (φ 0 (x)) β 0 d ds V (φ s(x))ds 0 (since d ds V (φ s(x)) 0) and therefore φ t (x) Ω β (φ t cannot exit B r since it would mean going through the boundary of B r ). 3. Since V is continuous, V (0) = 0 then there exists a δ > 0 such that x < δ V (x) < β. Therefore for x N δ x Ω β φ t (x) Ω β φ t (x) B r φ t (x) N ɛ (0). So for any ɛ > 0 we constructed a δ such that for all x N δ (0) and t 0, we have φ t (x) N ɛ (0), and therefore the origin is stable. Part (b) Note: Intuitively, condition V (x) < 0, x 0 s(i.e., V (x) is strictly decreasing along the trajectories of 1) implies that as t increases, the trajectory moves into lower level sets of V (x). We just need to show that it eventually goes to 0. In part (a) we showed that the origin is stable. What we need to show is that 10

3 there exists a δ > 0 such that for all x N δ (0) we have lim t φ t (x) = 0, i.e., there exists a δ > 0 such that for all ɛ > 0, there exists a T > 0 such that for all x N δ (0) and t > T, φ t (x) < ɛ (or φ t (x) N ɛ (0)). But since we showed that for all ɛ > 0 we can construct β such that Ω β N ɛ (0), i.e., φ t (x) Ω β φ t (x) N ɛ (0). Therefore, it is sucient to show that for all x N δ (0) lim V (φ t(x)) = 0 t (why? because this means that for all β > 0 there exists a T > 0, such that for t > T, V (φ t (x)) < β, i.e. φ t (x) Ω β N ɛ (0).) Since V is a decreasing function along the trajectories (condition (b)) and bounded below, then lim V (φ t(x)) = c 0. t Assume c > 0. Let Ω c = {x B r V (x) c}. By continuity of V and V (0) = 0, there exists a d > 0, such that B d = {x R n x d} Ω c. Since lim t V (φ t (x)) = c, then φ t (x) lies outside of B d, i.e., φ t (x) lies in the compact set d x r, V achieves its maximum in this set. Let α = maxd x r V (x) > 0. We have for t > 0 V (φ t (x)) = ˆ t d V (φ 0 (x)) + 0 ds V (φ s(x))ds V (φ 0 (x)) αt V (φ t (x)) < 0 c < 0 eventually But we assumed c > 0, we have a contradiction. Part (c) Reverse time (i.e., take t = t) then one gets part (b). Remarks: V satisfying the conditions of the theorem is called a Lyapunov function. The theorem allows to determine the stability of the equilibrium point without explicitly solving the dierential equation. In a sense, since V (x) = DV (x)f(x) the method converts a dynamics problem (i.e. determining the behavior of the trajectories over time), into a algebraic one (i.e., verifying inequalities of the form F (x) > 0, where F is some continuous function F : R n R) One can think of the Lyapunov function as a generalization of the idea of the energy of a system. Then the method studies stability by looking at the rate of change of this measure of energy. See [1] for a more detailed treatment of Lyapunov functions and nonlinear stability. The method does not show how to nd a Lyapunov function V. Denition: The region of attraction (RoA) of an the equilibrium point at the origin for (1) is {x R n lim t φ t (x) = 0}. Ω β are subsets of the RoA. This way we have a procedure for estimating the RoA (by maximizing β). More on this later. 11

4 Example 1 (Perko 9.5 (a) [2] ) ẋ = x + y + xy ẏ = x y x 2 y 3 Lets try V = x 2 + y 2 V = 2x( x + y + xy) + 2y(x y x 2 = 2x 2 + 4xy 2y 2 2y 4 = 2(x y) 2 2y 4 < 0 y 3 ) So the origin is asymptotically stable. Example 2 Linear Harmonic Oscillator (spring mass) ẍ + kx = 0, k > 0. Is the origin stable? ẏ = kx Energy E(x, y) = P E + KE = 1 2 kx y2. This is a good candidate for a Lyapunov function, i.e. V (x, y) = E(x, y). Lets check: Let D = R 2. First V (0, 0) = 0 and V (x, y) > 0 for (x, y) D\(0, 0). So it is stable. = kxy ykx = 0 What happens if we add a damping term to the equation? First Jacobian A = ( 0 1 k 0 ẏ = kx ɛy 3 (1 + x 2 ) ), λ = ±i k, so linear analysis not useful. Using same V we get = kxy + y( kx ɛy 3 (1 + x 2 )) = ɛy 4 (1 + x 2 ) So it is stable for ɛ > 0. Can show, using LaSalle's Invariance Principle (coming up) that it is indeed asymptotically stable for ɛ > 0 and unstable for ɛ < Global Stability Theorem (Barbashin-Krasovskii): Suppose f C 1 (R n ) and that f(0) = 0. Suppose further that there exists a real valued function V C 1 (R n ) satisfying V (0) = 0 and V (x) > 0 if x x 0, and x V (x) V (x) < 0, x 0 12

5 then x = 0 is globally asymptotically stable. Remark: The additional condition x V (x) guarantees that the level sets of V (x) are bounded. Why? For any p R n, let c = V (p). Condition means that for any c > 0 there is r > 0 such that x > r V (x) > c (by denition; similar to denition of lim t f(t) = ). x > r V (x) > c means that if x Ω c (i.e., V (x) c) then x B r (i.e., x r), and therefore Ω c B r and therefore bounded. Example 3 (Strogatz [3]) Lets try V = x 2m + ay 2n ẋ = x + 2y 3 2y 4 ẏ = x y + xy V = 2mx 2m 1 ( x + 2y 3 2y 4 ) + 2any 2n 1 ( x y + xy) = 2mx 2m + 4mx 2m 1 y 3 4mx 2m 1 y 4 2any 2n 1 x + 2any 2n x 2any 2n = 2mx 2m 2any 2n + ( 4mx 2m 1 y 3 4mx 2m 1 y 4 2any 2n 1 x + 2any 2n x ) let m=1 = 2x 2 2any 2n + ( 4xy 3 4xy 4 2any 2n 1 x + 2any 2n x ) let n=2 = 2x 2 4ay 4 + ( 4xy 3 4xy 4 4ay 3 x + 4ay 4 x ) let a=1 = 2x 2 4y 4 So V = x 2 + y 4 would work, and the origin is globally asymptotically stable. LaSalle's Theorem (LaSalle's Invariance Principle): Let Ω E be a compact set that is positively invariant with respect to the ow of (1). Suppose that there exists a real valued function V C 1 (E) such that V (x) 0 in Ω. Let D 0 be the set of all points in Ω where V (x) = 0, and M the largest invariant set in D 0. Then every solution starting in Ω approaches M as t. Remarks: The Theorem does not require that V is positive denite ( V (x) > 0, x 0). If M = {(0, 0)} then Ω is in the RoA of the origin. One can compute invariant subsets of the RoA of the origin by using the following Lemma: Lemma: If there exist a continuously dierentiable real valued function V and β > 0, such that the level set Ω V,β = {x R n V (x) β} is bounded and V (0) = 0, V (x) > 0, x 0 V (x) < 0, x 0, x Ω V,β then Ω V,β is invariant and in the RoA of the origin. Example 4 (Harmonic Oscillator) Let ẍ + sin x = 0. a) Can you prove stability of the origin using linearization? Use an appropriate Lyapunov function to prove that the origin is a stable xed point. (b) Add a damping term ẍ + ɛẋ + sin x = 0. Study the stability of the origin for ɛ > 0. Solution: (a) ẏ = sin x 13

6 ( 0 1 Lets look at xed point (0, 0). Jacobian A = 1 0 ), λ = ±i, so linear analysis not useful. Energy E(x, y) = P E + KE = 1 cos x y2. This is a good candidate for a Lyapunov function, i.e., V (x, y) = E(x, y). Lets check: Let E = ( π, π) R. First V (0, 0) = 0 and V (x, y) > 0 for (x, y) D\(0, 0). So it is stable. (b) Using samev we get = (sin x)y y sin x = 0 ẏ = sin x ɛy = (sin x)y + y( sin x ɛ(1 x 2 )y) = ɛy 2 Take then V 0 and D 0 = {(x, y) R 2 V (x, y) = 0} = {(x, y) R 2 y = 0}. But since for (x, y) D 0 (y = 0), ẏ = sin x 0 for x kπ, k Z, the largest invariant subset of D 0 is M = {(kπ, 0), k = 0, ±1, ±2,...}. I.e., the theorem states that the solutions will converge to one of the xed points. If we choose E = ( π, π) R, then we get M = {(0, 0)} and therefore the origin is asymptotically stable. Additionally, any solution starting in Ω β = {(x, y) E V (x, y) β} will converge to 0. Ω β is an estimate of the RoA of the origin. References [1] H. K. Khalil. Nonlinear Systems. Prentice Hall, 3 rd edition, [2] L. Perko. Dierential Equations and Dynamical Systems. Springer, 3 rd edition, [3] S. H. Strogatz. Nonlinear Dynamics And Chaos: With Applications To Physics, Biology, Chemistry, And Engineering. Westview Press,

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