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STABILITY IN THE WHOLE OF INVARIANT SETS FOR NONAUTONOMOUS DIFFERENTIAL INCLUSION 1
© E. A. Panasenko
In this work we continue to study (see [1]) different kinds of stability of positively invariant sets for a family of nonautonomous differential inclusions generated by a topological dynamical system. In particular, we discuss the conditions under which a positively invariant set is stable in the whole.
Let there be given a topological dynamical system (£, ft) and a map F : £ xRn — comp(Rn). Consider (for every a E £) the differential inclusion
x E F(fta,x), t E R (1)
and the ««convexified» differential inclusion
x E coF( f ta,x). (2)
From now and on we assume that for every a E £ the function F( f ta, x) is upper semicontinuous on x, bounded and uniformly continuous on t E R, and every solution of inclusion (1) is defined for all t ^ 0.
To every point u = (a, X) E Q = £ xcomp(Rn) we put into correspondence the section S(t, u) of the integral funnel of inclusion (2) and a dynamical system (Q, gt), where gtu = (f ta, S(t, u)). Next, for the given continuous function a — M(a) E comp(Rn) we construct the set M = {u = (a,X) E Q : X C M(a)} and its r-neighborhood Mr = {u = (a,X) E Q : X C Mr(a)}, where Mr(a) is the r-neighborhood of the set M(a).
Definition 1. The set M is called: 1) positively invariant with respect to inclusion
(1), if gtM C M for all t ^ 0; 2) stably positively invariant with respect to inclusion (1), if M
is positively invariant and for every e > 0 there exists 5 > 0 such that gtMs C M£ for every t ^ 0; 3) stable in the whole with respect to inclusion (1) if it is stably positively invariant and the deviation d(S(t,a,X),M(fta)) of the integral funnel S(t,a,X) from the set M(fta) tends to zero as t ^ to for every initial set X E comp(Rn).
Denote Nr = {u = (a, x) E Mr : u E M} and let us have a continuous scalar function u V(u), where u = (a,x) E Mr.
Definition2. Function V is said to be a Lyapunov function (with respect to the set M), if V(u) = 0 for all u E dM and V(u) > 0 for all u E Nr; a Lyapunov function V is said to be definitely positive (with respect to the set M) if for every e E (0, r) there exists 5 > 0 such that V(u) ^ 5 for all u E dM£.
We shall say that a function u -— V(u) is locally lipschitz if for every a E £ and each § > 0 there exists l such that for any two points (ti, xi) E Q = {(t,x) E R xRn : \t\ ^ §, x E Mr(ft a) , i = 1, 2, the inequality \V(ftjla, xi) — V(ft2a, x2)\ ^ l(\ti — t2\ + \xi — x2\) takes place.
1The work in partially supported by RFBR (grant №07-01-00305).
Let r > 0 and let a function V : Mr — R be locally lipschitz. Then there exists the limit VO(u; q) = Umsup Vff 'a>'V + 5h) — Vf ^
(^,y,5)^(0,x,+0) 5
which is called the generalized derivative of function V at the point u = (a, x) in the direction
q = (r,h) E R x Rn (or the Clarke derivative, [2]). If q = (1, h), then Vp(u) = max Vo(u; q) is
heF (u)
said to be the derivative of V with respect to inclusion (1).
In the paper [1] it has been shown that the following statement is true.
Theorem 1. If there exists a locally lipschitz Lyapunov function V : Mr — R such that Vp(u) ^ 0 for all u E Nr, then the set M is positively invariant with respect to inclusion (1). If, in addition, V is definitely positive, then the set M is stably positively invariant with respect to inclusion (1).
Definition 3. (see [3]) A function u = (a,x) — V (u) E R is called infinite large (with respect to the set M) if for every R > 0 there exists r > 0 such that for all u E Mr the relation V(u) ^ R holds.
Let a > 0. Denote &a = {u = (a, x) E Q : V(u) = a}.
Definition 4. We shall say that the set Sa does not contain positive semitrajectories of inclusion (2) if for every u E Sa and each solution ^(t, u) of inclusion (2) one can find t > 0 such that V(gTu) = a.
In other words, Sa does not contain positive semitrajectories of inclusion (2), if for every u E Sa any dynamic t — gtu = (fta,^(t,u)), where p(t,u) is one of the solutions to inclusion
(2), starting in Sa at t = 0 leaves Sa in a finite time.
Theorem 2. Let £ be compact. If there exists a locally lipschitz definitely positive and infinite large function V : Q — R such that Vfi(u) ^ 0 for all u E M, and for every a > 0 the set Sa does not contain positive semitrajectories of inclusion (2), then M is stable in the whole with respect to inclusion (1).
From this theorem there are also derived the statements on uniform (with respect to initial time moment) stability in the whole of the given set (t,M(t)) E R x comp(Rn) with respect to the ordinary differential inclusion x E F(t,x) and controllable system x = f (t,x,u), u E U.
REFERENCES
1. Panasenko E. A., Tonkov E. L. Invariant and stably invariant sets for differential inclusions // Trudy MIAN. 2008. (to appear)
2. Clarke F. H. Optimization and nonsmooth analysis. New York: Wiley Interscience, 1983.
3. Barbashin E. A. Lyapunov functions. M.: Nauka, 1970. [Russian]
Panasenko Elena A.
Tambov State University
Russia, Tambov
e-mail: [email protected]
Поступила в редакцию 25 апреля 2007 г.
