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Abstract and Applied Analysis

Volume 2012 (2012), Article ID 481853, 15 pages

http://dx.doi.org/10.1155/2012/481853

## Forward Euler Solutions and Weakly Invariant Time-Delayed Systems

^{1}Department of Mathematics and Applied Mathematics, VA Commonwealth University, Richmond, Virginia 23284, USA^{2}Departamento de Matemáticas, Facultad Experimental de Ciencias, Universidad del Zulia, Apartado 526, Maracaibo, Edo Zulia, Venezuela

Received 12 September 2012; Revised 10 December 2012; Accepted 11 December 2012

Academic Editor: Qiji J. Zhu

Copyright © 2012 Norma L. Ortiz-Robinson and Vinicio R. Ríos. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper presents a necessary and sufficient condition for the weak invariance property of a time-delayed system parametrized by a differential inclusion. The aforementioned condition generalizes the well-known Hamilton-Jacobi inequality that characterizes weakly invariant systems in the nondelay setting. The forward Euler approximation scheme used in the theory of discontinuous differential equations is extended to the time-delayed context by incorporating the delay and tail functions featuring the dynamics. Accordingly, an existence theorem of weakly invariant trajectories is established under the extended forward Euler approach.

#### 1. Introduction

The main goal of this paper is to establish a characterization of the so-called weakly invariant property of a closed set with respect to the set of solutions of the time-delay differential inclusion Throughout, the prescribed delay function is nonnegative and continuous on the interval . The multifunction is endowed with the following structural hypotheses (SH). (i) is a nonempty, convex, and compact set for every . (ii)There are constants such that for each (iii) is almost upper semicontinuous, which in presence of the previous assumptions, is equivalent to the following condition: for every compact interval and each , there exists a closed set with Lebesgue measure , so that the graph is closed in . A solution (or trajectory) to (1.1) is an absolutely continuous function satisfying (1.1) for almost all , for some . A more precise description and assumptions on the elements defining the data are provided in the next section.

Time delay has been shown to appear in many deterministic dynamical systems (see for instance [1–4]). When this feature is present, its study is essential to understand important qualitative and quantitative aspects of the dynamics. Furthermore, ignoring an existing time delay may result in an ill-posed model [5]. Based upon the nature of the state space, time-delay systems can be investigated by means of either infinite ([6–8]) or finite dimensional models ([1, 9, 10]). In the present work, the considered dynamics evolve according to the differential inclusion (1.1), which fits in the category of finite dimensional models, since the delay component of its right-hand side is being identified with the pointwise history of its trajectories. This setting, we believe, will prove to be satisfactory to further study strong invariance properties and their possible interconnections with Hamilton-Jacobi theory for problems involving time-delayed differential inclusions on finite dimensional spaces.

A complete exposition on the invariance concept used in this paper for the nondelayed setting can be consulted in [6, 11]. We also refer the reader to the comprehensive work [12] where notions of approximate invariance are introduced and a complete set of criteria for the weak and strong invariance properties are obtained in infinite dimensional Hilbert spaces. Here we briefly recall the weak invariance terminology in terms of the time-delayed differential inclusion (1.1). Given , a closed set and a multifunction , the pair is said to be *weakly invariant* if for every initial condition and every *tail function * with , there is a solution to (1.1) satisfying on and for all .

The original motivation to study invariance properties rises from the qualitative theory of dynamical systems under ordinary differential equations. Nevertheless, their applicability branched out to other areas, including partial differential equations, dynamic optimization, and control theory (see [6, 11, 13–16]).

The literature centered upon the invariance topic itself is abundant. We highlight the primary works [17–20] given in the context of vector field differential equations and the references [6–8, 13, 21–24] in the differential inclusion setting. All of the aforementioned works provide either sufficient or necessary conditions for the weak invariance property. However, it is noteworthy to mention that to the best of the authors' knowledge, weak invariance for dynamics exhibiting a time-delay behavior have only been discussed in the references [6–8], with functional differential inclusions being employed to model the underlined dynamics. The conditions for weak invariance provided in [6–8] are of tangential type only and roughly speaking they assert that weak invariance is equivalent to having, at each , the corresponding set of velocities overlapping the *Bouligand cone of ** at *:
where means that converges to the point , which is identified as the initial condition of the weakly invariant trajectory of (1.1). The closed cone (1.4) coincides with the tangent space if is a smooth manifold, but of course encloses a more general tangency concept.

A condition that is dual with the previous and that has been revisited by several authors (see [6, 11, 12, 15–17, 22, 24, 25]) to characterize the weak invariance property in the nondelayed framework is the following (we recall here the autonomous version). The pair is weakly invariant if and only if
where
is the *Hamiltonian* of the multifunction and
is the *Proximal Normal Cone* to at .

The Hamiltonian inequality (1.5) has proven to be an important link between some of the invariance properties of nondelayed systems and the theory of generalized solutions of Hamilton-Jacobi equations (see [11, 16, 18, 22, 26, 27]). As it was pointed out previously, this Hamiltonian interpretation is not available for time-delayed systems. To our knowledge, the only results deriving a Hamilton-Jacobi equation for time-delayed systems involve a game-theoretic approach in which the dynamics are prescribed by a continuous differential equation [28]. However, it is well understood that a great deal of applications stemming from optimal control theory, including discontinuous differential equations models, can be reformulated in terms of the more general paradigm (1.1). Therefore, the investigation about the form and the vality of (1.5) under the context of the time-delay differential inclusion (1.1) is of natural interest and represents in particular the main goal of this article.

The remaining material of this paper has been organized in three additional sections. Section 2 introduces the extension of the forward Euler solution concept given in [29] to Cauchy problems with time-delay components and establishes the corresponding existence theorem. Section 3 reveals the connection between the new solution concept and the trajectories of the time-delayed differential inclusion (1.1). The final section (Section 4) contains our main result, which is a Hamiltonian characterization of the weak invariance property for time-delayed systems modeled by (1.1).

#### 2. Delayed Forward Euler Solutions

Motivated by the problem of asymptotic stabilization, Bressan introduced the forward Euler approximation scheme in [29] to provide a suitable class of generalized solutions for nondelayed discontinuous differential equations. The new category of solutions subsumes the one generated by the classic Euler polygonal approach and provides some topological advantages when compared with the classes of Krasovskii and Filippov solutions that are used in the theory of discontinuous feedback controls [30, 31]. The aim of this section is to extend Bressan's solution concept to a more general setting where the dynamics exhibit a time delay behavior. This will serve as the initial step to show that weakly invariant trajectories of (1.1) can be identified within a wider class of solutions of certain initial value problems given in terms of time-delayed feedback selections. In this context, we consider for and every the following *time-delayed Cauchy problem*:
where the *tail function * is Lebesgue measurable and bounded, the *delay function * is continuous and for some positive constants and , the vector field satisfies the following linear growth condition:
We start with a partition of the interval
where . As usual, the diameter of is defined by . Given an initial point and a set of outer perturbations , all of them in , we construct a *delayed forward Euler polygonal* as follows:
A function is a *delayed forward Euler solution* (DFES) to the initial value problem (2.1) if there are sequences , , for all , such that the corresponding sequence of delayed forward Euler polygonals converges uniformly to , as .

The existence of a DFES is established by proving the stabilization of the previous scheme for certain subsequences of delayed forward Euler polygonals. This is done with the help of the following two discrete versions of Gronwall's inequality.

Lemma 2.1. *Assume , are sequences of nonnegative real numbers having the property that for some **
Then the following estimate holds:
*

*Proof. *To prove the lemma we first show by induction that if (2.6) holds, then
In fact, since a sum with zero terms is defined as and the product of zero factors is defined as , inequality (2.6) implies , which is (2.8) for . Let and assume that (2.8) holds for . Using hypothesis (2.6) we can write
On the other hand, notice that (using induction again on )
The expansion of the last sum and product on the right hand side of (2.10) yields
The following estimate follows from the tautology and from taking (2.10) into (2.9) and then using (2.11):
The last inequality completes the induction. Finally, we use (2.8) to obtain the exponential estimate (2.7). For every it is clear that . Therefore,
which is precisely the sought-after inequality.

Lemma 2.2. *Let be nonnegative numbers satisfying
**
where , and . Then
*

*Proof. *This lemma is an exercise proposed in page 183 of [11].

It is noteworthy to mention that the existence of forward Euler solutions has been clarified by Bressan in the context of nondelay patchy vector fields by comparing the new set of solutions with the set of Carathéodory solutions (see Theorem , part *(iv)* in [29]). The following existence result contrasts with Bressan's approach by following the ideas from Theorem 1.7 in ([11], Chapter 4) and extending it to the time-delayed framework under the more general class of vector fields satisfying (2.2).

Theorem 2.3. *Suppose that satisfies the linear growth condition (2.2) and is otherwise arbitrary. Then, at least one DFES exists for the time-delayed Cauchy problem (2.1) and the restriction of any DFES on is Lipschitz. *

* Proof. *To relax the notation, we assume without loss of generality that . Let be a natural number. For the uniform partition of
let be the corresponding delayed forward Euler polygonal with outer perturbations , nodes , and delayed nodes , for . We assume and . Let be fixed. Due to the hypothesis on , for every we have
Let and choose an index such that . The definition of implies . Using this observation, (2.4), and (2.17) we can write
Let . Using the identifications
and applying Lemma 2.2 to the last inequality, we have
If we set and , the application of Lemma 2.1 on (2.20) leads to
and thus, all nodes lie in . By convexity the delayed forward Euler polygonal, including its tail , must lie completely within the ball . For inequality (2.17) then implies
Therefore, the restriction of on is Lipschitz of rank . If is a sequence of uniform partitions such that as , we can apply the theorem of Arzela-Ascoli to the family restricted on , which is equicontinuous and uniformly bounded, to obtain a subsequence that converges uniformly to a continuous function on . Notice that not only does inherit the Lipschitz constant on , but also . Finally, a DFES of (2.1) is obtained by appending and .

#### 3. Trajectories of Time-Delay Differential Inclusions

This section identifies a particular set of trajectories of the time-delay differential inclusion problem (TDDI) where the data above consists of the interval , a multifunction satisfying SH, a positive constant , an initial time , a continuous delay function , and a tail function that is assumed to be Lebesgue measurable and bounded. As in the nondelay case [11], we show that any DFES associated to selections of the multifunction are indeed trajectories of TDDI. The following lemma plays a key role in this regard. The proof is a slight modification of Lemma 1.1 in [32] and therefore it is omitted.

Lemma 3.1. *Suppose is a continuous function on and satisfies
**
where and is measurable and bounded by on . Then the following inequality holds:
*

The next theorem is the main result of this section. It generalizes Corollary 1.12 of ([11], Chapter 4) to the context of a time-delayed differential inclusion with trajectories defined on a semi-infinite interval. For this purpose, it is necessary to extend the concept of a DFES to semi-infinite intervals by considering selections of . Recall that a selection is a function satisfying for almost all and all . Under this extended domain, we agree that a function is a delayed forward Euler solution of the corresponding Cauchy problem for if for every the restriction of on is a DFES for in the regular sense. This definition becomes coherent under the same argument that is used to extend nondelayed Euler solutions: an appropriate sequence of delayed forward Euler polygonals on is constructed, then a convergent subsequence on is obtained for a fixed , a further convergent subsequence on , and so on (see [11]).

Theorem 3.2. *Let be a selection of as above. If is a DFES for the Cauchy problem (2.1) on , then is a trajectory of . *

* Proof. *Let be a sequence of delayed forward Euler polygonals that converge uniformly to and that coincide with on . Choose . Let be the partition of associated with , and for we define the following functions:
For any , it is readily seen that
which guarantees the measurability of . Moreover, and are continuous functions, whence the composites and turn out to be measurable, and so also do the linear combinations and .

If is the Lipschitz constant for on , then the following estimates hold:
Thus both converge to zero in and converges to a.e. Since is a selection of , at every nonpartition point we have
Therefore, for almost all the linear growth condition on implies
We now apply Gronwall's inequality ([11], page 179) on (3.8), followed by Lemma 3.1 with , , and to obtain
The last inequality leads to a uniform bound for and hence for due to (3.8). Given the theorem of Arzela-Ascoli and the weak compactness in , we may choose converging subsequences of and , respectively (without relabeling), so that
A simple variant of the sequential compactness result given in Chapter 3 of [11] that holds under SH can be applied to obtain that is a trajectory of on . Repeating the argument described above on with the subsequence shows that is a trajectory of on . We may continue in this fashion to establish the desired property for on .

#### 4. The Main Result

The following observation provides a sufficient condition for delayed forward Euler solutions of (2.1) to approach a closed set . It plays in particular a crucial role in the main contribution of this article (Theorem 4.2 below) by linking the existence result given in Theorem 3.2 and the weak invariance property of TDDI.

Theorem 4.1. *Suppose satisfies (2.2) and let be a DFES for on . Let be an open set containing for all and suppose that for every there exists such that . Then one has
*

*Proof. *Let be one element from the sequence of delayed forward Euler polygonals converging uniformly to and let be its associated partition. Let be the external perturbations defining and as in the previous result, we denote by and their associated nodes and delayed nodes. Given , due to the uniform convergence we may assume that lies completely in and for . The hypothesis asserts that for each there is a point such that . If is as in (2.22), then the following estimates hold:
The recursive application of last inequality yields
Let . If , then . From this last and (4.3) we have
We now apply Lemma 2.1 with to obtain
The previous estimates holds at any node and then after passing to the limit when we obtain
which in turn implies
since is arbitrary.

In view of the previous result, weakly invariant trajectories for TDDI are naturally identified as delayed forward Euler solutions of particular selections of . Accordingly, the Hamiltonian (1.5) plays an important role in the design of these selections and features the following weakly invariant criterion, which is the main contribution of this paper.

Theorem 4.2. *Let be closed. Under the assumptions of TDDI, the pair is weakly invariant if and only if
**
In this case, (4.8) holds at all points of density of a certain countable family of pairwise disjoint closed sets (), for which is upper semicontinuous on each . *

*Remarks 1. *Recall that a point is a point of density of a measurable set if
where denotes the Lebesgue measure in . The set of all points of density of is denoted by and has full measure in . Furthermore, if , then the equality holds for any null set . It is clear that if is closed, then (see [33]).

*Proof. *Let be the set of full measure such that (4.8) holds for all . For every we choose an element and then select so that
We extend , if necessary, by setting for all . It follows that the multifunction induces the linear growth property (2.2) on and also for all . Consequently, if is a DFES of the problem a.e., , with , Theorem 4.1 implies for all . We may extend on by considering the previous Cauchy problem on the interval and so on. It turns out that the resulting arc is a solution to TDDI. In fact, define
Then satisfies the standing hypotheses and we have when . It is clear that for almost all and all . Therefore, Theorem 3.2 implies that is a trajectory of . Since for all , it follows that and hence is also a trajectory of .

To prove the converse, let us consider and recall that the almost upper semicontinuity of implies the existence of a countable family of pairwise disjoint closed sets such that is upper semicontinuous on for all and the union has full measure in (see page 29 in [34] or page 24 in [35]). Let denote the points of density of , and define , which has null measure, and take for some . Let be a sequence in satisfying and set . By hypothesis, there is a weakly invariant solution to TDDI such that . The absolute continuity of , the compact-convex valuedness of and the upper semicontinuity of on , lead to the existence of a subsequence , which we relabel as , such that
On the other hand, recall that for all . Therefore, for any the definition (1.7) asserts the existence of such that
which implies the Hamiltonian inequality in the statement at the point . Finally, we repeat the previous argument on each of the intervals to obtain that (4.8) is satisfied almost everywhere on the whole interval .

#### References

- W. C. Chen, “Dynamics and control of a financial system with time-delayed feedbacks,”
*Chaos, Solitons and Fractals*, vol. 37, no. 4, pp. 1198–1207, 2008. View at Publisher · View at Google Scholar · View at Scopus - H. Gorceki, S. Fuksa, P. Grabowski, and A. Korytowski,
*Analysis and Synthesis of Time Delay Systems*, John Wiley & Sons, New York, NY, USA, 1989. - Y. Lenbury and P. Pornsawad, “A delay-differential equation model of the feedback-controlled hypothalamus-pituitary-adrenal axis in humans,”
*Mathematical Medicine and Biology*, vol. 22, no. 1, pp. 15–33, 2005. View at Publisher · View at Google Scholar · View at Scopus - G. Rosen, “Time delays produced by essential nonlinearity in population growth models,”
*Bulletin of Mathematical Biology*, vol. 49, no. 2, pp. 253–255, 1987. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - A. S. Matveev, “The instability of optimal control problems to time delay,”
*SIAM Journal on Control and Optimization*, vol. 43, no. 5, pp. 1757–1786, 2005. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - J.-P. Aubin,
*Viability Theory*, Birkhäuser, Boston, Mass, USA, 1991. View at Zentralblatt MATH · View at MathSciNet - G. Haddad, “Monotone trajectories of differential inclusions and functional-differential inclusions with memory,”
*Israel Journal of Mathematics*, vol. 39, no. 1-2, pp. 83–100, 1981. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - G. Haddad, “Monotone viable trajectories for functional-differential inclusions,”
*Journal of Differential Equations*, vol. 42, no. 1, pp. 1–24, 1981. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - M. Basin, J. Rodriguez-Gonzalez, and R. Martinez-Zuniga, “Optimal control for linear systems with time delay in control input based on
the duality principle,” in
*Proceedings of the American Control Conference (ACC '03)*, pp. 2144–2148, Denver, Colo, USA, June 2003. View at Scopus - R. D. Driver,
*Ordinary and Delay Differential Equations*, Springer-Verlag, New York, NY, USA, 1977. View at MathSciNet - F. H. Clarke, Yu. S. Ledyaev, R. J. Stern, and P. R. Wolenski,
*Nonsmooth Analysis and Control Theory*, vol. 178, Springer-Verlag, New York, NY, USA, 1998. View at MathSciNet - F. H. Clarke, Yu. S. Ledyaev, and M. L. Radulescu, “Approximate invariance and differential inclusions in Hilbert spaces,”
*Journal of Dynamical and Control Systems*, vol. 3, no. 4, pp. 493–518, 1997. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - J.-P. Aubin, “Viability solutions to structured Hamilton-Jacobi equations under constraints,”
*SIAM Journal on Control and Optimization*, vol. 49, no. 5, pp. 1881–1915, 2011. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - F. H. Clarke,
*Optimization and Nonsmooth Analysis*, vol. 5, Society for Industrial and Applied Mathematics (SIAM), Philadelphia, Pa, USA, 1990. View at Publisher · View at Google Scholar · View at MathSciNet - H. Frankowska, “Lower semicontinuous solutions of Hamilton-Jacobi-Bellman equations,”
*SIAM Journal on Control and Optimization*, vol. 31, no. 1, pp. 257–272, 1993. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - A. I. Subbotin, “A generalization of the basic equation of the theory of differential games,”
*Soviet Mathematics*, vol. 22, pp. 358–362, 1980. - J.-M. Bony, “Principe du maximum, inégalite de Harnack et unicité du problème de Cauchy pour les opérateurs elliptiques dégénérés,”
*Université de Grenoble. Annales de l'Institut Fourier*, vol. 19, no. 1, pp. 277–304, 1969. View at MathSciNet - M. G. Crandall, “A generalization of Peano's existence theorem and flow invariance,”
*Proceedings of the American Mathematical Society*, vol. 36, pp. 151–155, 1972. View at Zentralblatt MATH · View at MathSciNet - M. Nagumo, “Uber die Lage der Integralkurven gewöhnlicher Differentialgleichungen,”
*Proceedings of the Physico-Mathematical Society of Japan*, vol. 24, pp. 551–559, 1942. View at Zentralblatt MATH · View at MathSciNet - R. M. Redheffer and W. Walter, “Flow-invariant sets and differential inequalities in normed spaces,”
*Applicable Analysis*, vol. 5, no. 2, pp. 149–161, 1975. View at Zentralblatt MATH · View at MathSciNet - F. H. Clarke, Yu. S. Ledyaev, R. J. Stern, and P. R. Wolenski, “Qualitative properties of trajectories of control systems: a survey,”
*Journal of Dynamical and Control Systems*, vol. 1, no. 1, pp. 1–48, 1995. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - H. Frankowska, S. Plaskacz, and T. Rzeżuchowski, “Measurable viability theorems and the Hamilton-Jacobi-Bellman equation,”
*Journal of Differential Equations*, vol. 116, no. 2, pp. 265–305, 1995. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Yu. S. Ledyaev, “Criteria for viability of trajectories of nonautonomous differential inclusions and their applications,”
*Journal of Mathematical Analysis and Applications*, vol. 182, no. 1, pp. 165–188, 1994. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - V. M. Veliov, “Sufficient conditions for viability under imperfect measurement,”
*Set-Valued Analysis*, vol. 1, no. 3, pp. 305–317, 1993. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - T. Donchev, V. Ríos, and P. Wolenski, “Strong invariance and one-sided Lipschitz multifunctions,”
*Nonlinear Analysis. Theory, Methods & Applications*, vol. 60, no. 5, pp. 849–862, 2005. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - V. R. Ríos and P. R. Wolenski, “Proximal characterization of the reachable set for a discontinuous differential inclusion,” in
*Geometric Control and Nonsmooth Analysis*, F. Ancona, A. Bressan, P. Cannarsa, F. H. Clarke, and P. R. Wolenski, Eds., vol. 76, pp. 270–279, Worldscientific, Singapore, 2008. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - A. I. Subbotin,
*Generalized Solutions of First-Order PDEs: The Dynamical Optimization Perspective*, Birkhauser, Boston, Ma, USA, 1995. View at MathSciNet - N. Yu. Lukoyanov, “On Hamilton-Jacobi formalism in time-delay control systems,”
*Trudy Instituta Matematiki i Mekhaniki UrO RAN*, vol. 16, no. 5, pp. 269–277, 2010. - A. Bressan, “Singularities of stabilizing feedbacks,”
*Rendiconti del Seminario Matematico dell Úniversita e del Politecnico di Torino*, vol. 56, no. 4, pp. 87–104, 1998. View at Zentralblatt MATH · View at MathSciNet - J.-M. Coron and L. Rosier, “A relation between continuous time-varying and discontinuous feedback stabilization,”
*Journal of Mathematical Systems, Estimation, and Control*, vol. 4, no. 1, pp. 67–84, 1994. View at Zentralblatt MATH · View at MathSciNet - E. P. Ryan, “On Brockett's condition for smooth stabilizability and its necessity in a context of nonsmooth feedback,”
*SIAM Journal on Control and Optimization*, vol. 32, no. 6, pp. 1597–1604, 1994. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - X. Xiang and H. Kuang, “Optimal control of time-delay systems,”
*Journal of Mathematics and System Science*, vol. 13, no. 1, pp. 31–41, 2000. - E. Hewitt and K. Stromberg,
*Real and Abstract Analysis*, Springer-Verlag, New York, NY, USA, 1975. View at MathSciNet - D. Bothe, “Multivalued differential equations on graphs,”
*Nonlinear Analysis. Theory, Methods & Applications*, vol. 18, no. 3, pp. 245–252, 1992. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - K. Deimling,
*Multivalued Differential Equations*, vol. 1, Walter de Gruyter & Co., Berlin, Germany, 1992. View at Publisher · View at Google Scholar · View at MathSciNet