1. COMMUTATION RELATIONS and STABILITY of SWITCHED SYSTEMS Daniel Liberzon Coordinated Science Laboratory and Dept. of Electrical & Computer Eng., Univ. of Illinois at Urbana-Champaign

2. COMMUTING STABLE MATRICES => GUES For subsystems – similarly (commuting Hurwitz matrices)

3. ...... quadratic common Lyapunov function [ Narendra–Balakrishnan ’94 ] COMMUTING STABLE MATRICES => GUES Alternative proof: is a common Lyapunov function

4. Nilpotent means sufficiently high-order Lie brackets are 0 NILPOTENT LIE ALGEBRA => GUES Lie algebra: Lie bracket: Hence still GUES [ Gurvits ’95 ] For example: (2nd-order nilpotent) In 2nd-order nilpotent case Recall: in commuting case

5. SOLVABLE LIE ALGEBRA => GUES Example: quadratic common Lyap fcn diagonal exponentially fast [ Kutepov ’82, L–Hespanha–Morse ’99 ] Larger class containing all nilpotent Lie algebras Suff. high-order brackets with certain structure are 0 exp fast Lie’s Theorem: is solvable triangular form

6. MORE GENERAL LIE ALGEBRAS Levi decomposition: radical (max solvable ideal) There exists one set of stable generators for which gives rise to a GUES switched system, and another which gives an unstable one [ Agrachev–L ’01 ] is compact (purely imaginary eigenvalues) GUES, quadratic common Lyap fcn is not compact not enough info in Lie algebra:

7. SUMMARY: LINEAR CASE Lie algebra w.r.t. Assuming GES of all modes, GUES is guaranteed for: commuting subsystems: nilpotent Lie algebras (suff. high-order Lie brackets are 0) e.g. solvable Lie algebras (triangular up to coord. transf.) solvable + compact (purely imaginary eigenvalues) Further extension based only on Lie algebra is not possible Quadratic common Lyapunov function exists in all these cases

8. SWITCHED NONLINEAR SYSTEMS Lie bracket of nonlinear vector fields: Reduces to earlier notion for linear vector fields (modulo the sign)

9. SWITCHED NONLINEAR SYSTEMS Linearization (Lyapunov’s indirect method) Can prove by trajectory analysis [ Mancilla-Aguilar ’00 ] or common Lyapunov function [ Shim et al. ’98, Vu–L ’05 ] Global results beyond commuting case – ? [ Unsolved Problems in Math. Systems & Control Theory ’04 ] Commuting systems GUAS

10. SPECIAL CASE globally asymptotically stable Want to show: is GUAS Will show: differential inclusion is GAS

11. OPTIMAL CONTROL APPROACH Associated control system: where (original switched system ) Worst-case control law [Pyatnitskiy, Rapoport, Boscain, Margaliot] : fix and small enough

12. MAXIMUM PRINCIPLE is linear in at most 1 switch (unless ) GAS Optimal control: (along optimal trajectory)

13. SINGULARITY Need: nonzero on ideal generated by (strong extremality) At most 2 switches GAS Know: nonzero on strongly extremal (time-optimal control for auxiliary system in ) constant control (e.g., ) Sussmann ’79:

14. GENERAL CASE GAS Want: polynomial of degree (proof – by induction on ) bang-bang with switches

15. THEOREM Suppose: GAS, backward complete, analytic s.t. and Then differential inclusion is GAS, and switched system is GUAS [ Margaliot–L ’06 ] Further work in [ Sharon–Margaliot ’07 ]

16. REMARKS on LIE-ALGEBRAIC CRITERIA Checkable conditions In terms of the original data Independent of representation Not robust to small perturbations In any neighborhood of any pair of matrices there exists a pair of matrices generating the entire Lie algebra [ Agrachev–L ’01 ] How to measure closeness to a “nice” Lie algebra?