Identification of one-parameter bifurcations giving rise to periodic orbits, from their period function

Transcription

1 Identification of one-parameter bifurcations giving rise to periodic orbits, from their period function Armengol Gasull 1, Víctor Mañosa 2, and Jordi Villadelprat 3 1 Departament de Matemàtiques Universitat Autònoma de Barcelona 2 Departament de Matemàtica Aplicada III Control, Dynamics and Applications Group (CoDALab) Universitat Politècnica de Catalunya. 3 Departament d Enginyeria Informàtica i Matemàtiques Universitat Rovira i Virgili. 15th International Workshop on Dynamics and Control. May 31 June 3, 2009, Tossa de Mar, Spain. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 1 / 18

2 1. MOTIVATION Suppose that we have a model for a realistic phenomenon ẋ = X µ (x) = X(x; µ, λ), x R n, where Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 2 / 18

3 1. MOTIVATION Suppose that we have a model for a realistic phenomenon ẋ = X µ (x) = X(x; µ, λ), x R n, where µ R is tuning parameter (experimentally controllable). Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 2 / 18

4 1. MOTIVATION Suppose that we have a model for a realistic phenomenon ẋ = X µ (x) = X(x; µ, λ), x R n, where µ R is tuning parameter (experimentally controllable). and λ R p are uncertain parameters that need to be estimated. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 2 / 18

5 1. MOTIVATION Suppose that we have a model for a realistic phenomenon ẋ = X µ (x) = X(x; µ, λ), x R n, where µ R is tuning parameter (experimentally controllable). and λ R p are uncertain parameters that need to be estimated. If it is possible to measure the period T (µ) of some periodic orbits observed experimentally, and its evolution as µ varies. Then perhaps it is possible to extract information on the uncertain parameters λ from T (µ)? Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 2 / 18

6 This problem arises when studying neuron activities in the brain with the aim of determining the synaptic conductances λ that it receives. By injecting external currents µ, in the neurons it is possible to extract information about the period of the oscillations of the voltage of the cell T (µ). Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 3 / 18

7 This problem arises when studying neuron activities in the brain with the aim of determining the synaptic conductances λ that it receives. By injecting external currents µ, in the neurons it is possible to extract information about the period of the oscillations of the voltage of the cell T (µ). From measurements T (µ i, λ) for i = 1,..., q some kind of regression is needed to estimate λ. The analytical knowledge of T (µ, λ) is an advantage to do this regression. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 3 / 18

8 2. STARTING POINT From the knowledge T (µ) of a one parameter family of P.O. it is possible to identify the type of bifurcation which has originated the P.O.? YES important restrictions on the uncertainties λ R p to be estimated better knowledge of the model. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 4 / 18

9 2. STARTING POINT From the knowledge T (µ) of a one parameter family of P.O. it is possible to identify the type of bifurcation which has originated the P.O.? YES important restrictions on the uncertainties λ R p to be estimated better knowledge of the model. Our results are restricted to the planar analytic case where the dependence of the differential equation on µ is also analytic. { ẋ = P(x, y; µ), or equivalently X(x, y; µ) = P(x, y; µ) + Q(x, y; µ) ẏ = Q(x, y; µ). x y (1) where (x, y) R 2 and µ Λ R an open interval containing zero. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 4 / 18

10 2. STARTING POINT From the knowledge T (µ) of a one parameter family of P.O. it is possible to identify the type of bifurcation which has originated the P.O.? YES important restrictions on the uncertainties λ R p to be estimated better knowledge of the model. Our results are restricted to the planar analytic case where the dependence of the differential equation on µ is also analytic. { ẋ = P(x, y; µ), or equivalently X(x, y; µ) = P(x, y; µ) + Q(x, y; µ) ẏ = Q(x, y; µ). x y (1) where (x, y) R 2 and µ Λ R an open interval containing zero. Our objective is to relate the form of T (µ) with the type of bifurcation of limit cycles. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 4 / 18

11 2. MOST ELEMENTARY BIFURCATIONS The most elementary bifurcations of planar vector fields of the above form occur when the the vector field X(x, y; µ) = P(x, y; µ) x has a first degree of structural instability. + Q(x, y; µ) y Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 5 / 18

12 2. MOST ELEMENTARY BIFURCATIONS The most elementary bifurcations of planar vector fields of the above form occur when the the vector field X(x, y; µ) = P(x, y; µ) x has a first degree of structural instability. Intuitively + Q(x, y; µ) y Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 5 / 18

13 All the possible bifurcations that can occur for a vector field with a 1st degree of structural instability are known (Andronov et al and Sotomayor 1974) Among them we only study the isolated ones that give rise to isolated periodic orbits (P.O.). Elementary bifurcations giving rise to P.O. (a) Hopf bifurcation. (b) Bifurcation from semi stable periodic orbit. (c) Saddle-node bifurcation (d) Saddle loop bifurcation. We will characterize the asymptotic expansion of the period of the emerging P.O. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 6 / 18

14 3. SUMMARY OF OUR RESULTS Theorem : Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 7 / 18

15 3. SUMMARY OF OUR RESULTS Theorem : Generically the form of T (µ) characterizes the type of bifurcation Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 7 / 18

16 3. SUMMARY OF OUR RESULTS Theorem : Generically the form of T (µ) characterizes the type of bifurcation Under the conditions that give rise to a bifurcation of P.O. we have Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 7 / 18

17 3. SUMMARY OF OUR RESULTS Theorem : Generically the form of T (µ) characterizes the type of bifurcation Under the conditions that give rise to a bifurcation of P.O. we have Hopf bifurcation: T (µ) = T 0 + T 1 µ + O( µ 3/2 ), with T 0 > 0 but T 1 can be 0. Bifurcation from semi stable periodic orbit: T ± (µ) = T 0 ± T 1 µ + O(µ), with T0 > 0 but T 1 can be 0. Saddle-node bifurcation: T (µ) T 0 / µ ( ) Saddle loop bifurcation: T (µ) = c ln µ + O(1), with c 0. ( ) T (µ) a Where T (µ) a + f (µ) means that lim = 1. µ 0 f (µ) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 7 / 18

18 HOPF BIFURCATION It is originated by the change of the stability of the equilibrium Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 8 / 18

19 HOPF BIFURCATION It is originated by the change of the stability of the equilibrium Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 8 / 18

20 HOPF BIFURCATION It is originated by the change of the stability of the equilibrium Theorem. Under the conditions that give rise to a bifurcation of P.O. we have with T 0 > 0 but T 1 can be 0. T (µ) = T 0 + T 1 µ + O( µ 3/2 ) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 8 / 18

21 BIFURCATION FROM SEMI STABLE PERIODIC ORBIT...my preferred one. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 9 / 18

22 BIFURCATION FROM SEMI STABLE PERIODIC ORBIT...my preferred one. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 9 / 18

23 BIFURCATION FROM SEMI STABLE PERIODIC ORBIT...my preferred one. Theorem. Under the conditions that give rise to a bifurcation of P.O. we have T ± (µ) = T 0 ± T 1 µ + O(µ) with T 0 > 0 but T 1 can be 0. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 9 / 18

24 SADDLE NODE BIFURCATION The bifurcation takes place when a connected saddle and a node collapse. Theorem. Under the conditions that give rise to a bifurcation of P.O. we have T (µ) T 0 / µ Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 10 / 18

25 SADDLE LOOP BIFURCATION Occurs when a saddle loop breaks and the separatrices change their relative position Theorem. Under the conditions that give rise to a bifurcation of P.O. we have with c 0. T (µ) = c ln µ + O(1) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 11 / 18

26 4. WHAT ABOUT THE PROOFS? (a) Hopf bifurcation. Usual arguments using polar coordinates, Taylor developments, and variational equations. (b) Bifurcation from semi stable periodic orbit. Try to reduce the problem to the Hopf bifurcation framework, but instead of using polar coordinates using specific local coordinates (Ye et al. 1983) (c) Saddle-node bifurcation. Work with normal form theory (Il yashenko, Li. 1999) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 12 / 18

27 4. WHAT ABOUT THE PROOFS? (a) Hopf bifurcation. Usual arguments using polar coordinates, Taylor developments, and variational equations. (b) Bifurcation from semi stable periodic orbit. Try to reduce the problem to the Hopf bifurcation framework, but instead of using polar coordinates using specific local coordinates (Ye et al. 1983) (c) Saddle-node bifurcation. Work with normal form theory (Il yashenko, Li. 1999) (d) Saddle loop bifurcation. Usual arguments+normal forms but the framework is very different because the structure of the transition maps of the flow, and the transit time functions are NON DIFFERENTIABLE! Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 12 / 18

28 SKETCH OF THE PROOF IN THE SADDLE LOOP CASE (A) LOCATION. A periodic orbit can be seen as a zero of the displacement function D(s; µ) = P 1 (s; µ) P 2 (s; µ) So the P.O. are located by curve s l (µ) such that D(s l (µ); µ) = 0. We would like to apply the implicit function theorem, but... P 1 (s) is not differentiable. (Roussarie, 1998) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 13 / 18

29 (B) TRANSIT TIME ANALYSIS. The flying time for any orbit can be decomposed as T (s; µ) = T 1 (s; µ) + T 2 (s; µ) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 14 / 18

30 (B) TRANSIT TIME ANALYSIS. The flying time for any orbit can be decomposed as T (s; µ) = T 1 (s; µ) + T 2 (s; µ) So the period of each limit cycle is given by T (µ) = T (s l (µ); µ) = T 1 (s l (µ); µ) + T 2 (s l (µ); µ) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 14 / 18

31 (B) TRANSIT TIME ANALYSIS. The flying time for any orbit can be decomposed as T (s; µ) = T 1 (s; µ) + T 2 (s; µ) So the period of each limit cycle is given by T (µ) = T (s l (µ); µ) = T 1 (s l (µ); µ) + T 2 (s l (µ); µ) But... T 1 (s) is also non differentiable. (Broer, Roussarie, Simó, 1996) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 14 / 18

32 To study P 1 (s) and T 1 (s) we need Normal forms Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 15 / 18

33 To study P 1 (s) and T 1 (s) we need Normal forms Lemma. Consider the family {X µ} µ Λ, such that for µ 0 Λ, X µ0 has a saddle connection at p µ0. Set r(µ) = λ 1 (µ)/λ 2 (µ), the ratio of the eigenvalues of the saddle. For any k N there exists a C k diffeomorphism Φ (also depending C k on µ) such that, in some neighbourhood of (p µ0, µ 0 ) R 2 Λ: (a) If r(µ 0 ) / Q, X µ = Φ ( λ 1 (µ) x x + λ 2 (µ) y y ) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 15 / 18

34 To study P 1 (s) and T 1 (s) we need Normal forms Lemma. Consider the family {X µ} µ Λ, such that for µ 0 Λ, X µ0 has a saddle connection at p µ0. Set r(µ) = λ 1 (µ)/λ 2 (µ), the ratio of the eigenvalues of the saddle. For any k N there exists a C k diffeomorphism Φ (also depending C k on µ) such that, in some neighbourhood of (p µ0, µ 0 ) R 2 Λ: (a) If r(µ 0 ) / Q, X µ = Φ ( λ 1 (µ) x x + λ 2 (µ) y y ) (b) If r(µ 0 ) = p/q ( )) 1 X µ = Φ (x x + yg(u; µ) y, f (u; µ) where f (u; µ) and g(u; µ) are polynomials in u := x p y q with coefficients C functions in µ. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 15 / 18

35 Proposition. (a) If r(0) > 1 then P 1 (s; µ) =s r(µ)( 1 + ψ 1 (s; µ) ) and T 1 (s; µ) = 1 λ 1 (µ) ln s+ψ 2(s; µ) Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 16 / 18

36 Proposition. (a) If r(0) > 1 then P 1 (s; µ) =s r(µ)( 1 + ψ 1 (s; µ) ) and T 1 (s; µ) = 1 λ 1 (µ) ln s+ψ 2(s; µ) (b) If r(0) = 1 then, P 1 (s; µ) =s (1 r(µ) + α 2 (µ)s ω ( s; α 1 (µ) ) ) +ψ 1 (s; µ), and T 1 (s; µ) = 1 λ 1 (µ) ln s+β 1(µ)s ω ( s; α 1 (µ) ) +ψ 2 (s; µ). Where α 1 (µ) = 1 r(µ), and α 2 and β 1 are C. Where ψ i belong to a class of functions B with good behaviour at (0, µ 0 ), and ω is the Roussarie Ecalle compensator. Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 16 / 18

37 The Roussarie Ecalle compensator. It is a function which captures the non regular behaviour of the so called Dulac maps. Definition. The function defined for s > 0 and α R by means of { s α 1 ω(s; α) = α if α 0, ln s if α = 0, is called the Roussarie-Ecalle compensator. See (Roussarie, 1998). Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 17 / 18

38 References Andronov, Leontovich, Gordon, Maier, Theory of bifurcation of dynamic systems on a plane, John Wiley & Sons, New York, Broer, Roussarie, Simó, Invariant circles in the Bogdanov Takens bifurcation for diffeomorphisms, Ergodic Th. Dynam. Sys. 16 (1996), Il yashenko, Li, Nonlocal bifurcations, Mathematical Surveys and Monographs 66, American Mathematical Society, Providence, RI, Roussarie, Bifurcations of planar vector fields and Hilbert s sixteenth problem, Progr. Math. 164, Birkhäuser. Basel, Sotomayor, Generic one parameter families of vector fields on two dimensional manifolds. Publ. Math. IHES, 43 (1974), Ye et al. Theory of limit cycles, Translations of Math. Monographs 66, American Mathematical Society, Providence, RI, The picture in Slide 4 comes from Edgerton Simulating in vivo-like Synaptic Input Patterns in Multicompartmental Models. THANK YOU! Gasull,Mañosa,Villadelprat (UAB-UPC-URV) Identification of Bifurcations Dynamics & Control 18 / 18