Existence of Solution and Approximate Controllability for Neutral Differential Equation with State Dependent Delay
Abstract
This paper is divided in two parts. In the first part we study a second order neutral partial differential equation with state dependent delay and noninstantaneous impulses. The conditions for existence and uniqueness of the mild solution are investigated via Hausdorff measure of noncompactness and Darbo Sadovskii fixed point theorem. Thus we remove the need to assume the compactness assumption on the associated family of operators. The conditions for approximate controllability are investigated for the neutral second order system with respect to the approximate controllability of the corresponding linear system in a Hilbert space. A simple range condition is used to prove approximate controllability. Thereby, we remove the need to assume the invertibility of a controllability operator used by authors in (Balachandran and Park, 2003), which fails to exist in infinite dimensional spaces if the associated semigroup is compact. Our approach also removes the need to check the invertibility of the controllability Gramian operator and associated limit condition used by the authors in (Dauer and Mahmudov, 2002), which are practically difficult to verify and apply. Examples are provided to illustrate the presented theory.
1. Introduction
Neutral differential equations appear as mathematical models in electrical networks involving lossless transmission, mechanics, electrical engineering, medicine, biology, ecology, and so forth. Neutral differential equations are functional differential equations in which the highest order derivative of the unknown function appears both with and without derivatives. Second order neutral differential equations model variational problems in calculus of variation and appear in the study of vibrating masses are attached to an electric bar.
Impulsive differential equations are known for their utility in simulating processes and phenomena subject to short term perturbations during their evolution. Discrete perturbations are negligible to the total duration of the process which have been studied in [1–6].
However noninstantaneous impulses are recently studied by Ahmad [7]. Stimulated by their numerous applications in mechanics, electrical engineering, medicine, ecology, and so forth, noninstantaneous impulsive differential equations are recently investigated.
Recently, much attention is paid to partial functional differential equation with state dependent delay. For details see [7–12]. As a matter of fact, in these papers their authors assume severe conditions on the operator family generated by A, which imply that the underlying space X has finite dimension. Thus the equations treated in these works are really ordinary and not partial equations. The literature related to state dependent delay mostly deals with functional differential equations in which the state belongs to a finite dimensional space. As a consequence, the study of partial functional differential equations with state dependent delay is neglected. This is one of the motivations of our paper.
The papers [13, 14] study existence of differential equation via measure of noncompactness. Measure of noncompactness significantly removes the need to assume Lipschitz continuity of nonlinear functions and operators.
In recent years, controllability of infinite dimensional systems has been extensively studied for various applications. In the papers [15, 16] the authors discuss the exact controllability results by assuming that the semigroup associated with the linear part is compact. However, if the operator B is compact or C0-semigroup T(t) is compact then the controllability operator is also compact. Hence the inverse of it does not exist if the state space X is infinite dimensional [17].
Another available method in the literature involves the invertibility of operator , where is the controllability Gramian and a limit condition which is difficult to check and apply in practical real world problems. See for details [18]. Also it is practically difficult to verify their condition directly. This is one of the motivations of our paper.
However our work is a continuation of coauthor Sukavanam′s novel approach in article [19]. We extend our work [20–22] in this paper.
Controllability results are available in overwhelming majority for abstract differential delay systems (see [1, 3–6, 9–12, 14–17, 19–34]), rather than for neutral differential with state dependent delay.
2. Preliminaries
- (a)
C(0) = I (I is the identity operator in X);
- (b)
C(t + s) + C(t − s) = 2C(t)C(s) for all ;
- (c)
the map t → C(t)x is strongly continuous for each x ∈ X.
The operator A is the infinitesimal generator of a strongly continuous cosine function of bounded linear operators and S(t) is the associated sine function. Let N, be certain constants such that ∥C(t)∥ ≤ N and for every t ∈ J = [0, a]. For more details see book by Fattorini [28] and articles [35–37]. In this work we use the axiomatic definition of phase space , introduced by Hale and Kato [30].
Definition 1 (see [30].)Let be a linear space of functions mapping (−∞, 0] into X endowed with seminorm and satisfy the following conditions:
- (A)
If x : (−∞, σ + b] → X, b > 0, such that and x|[σ,σ+b] ∈ C([σ, σ + b] : X), then for every t ∈ [σ, σ + b) the following conditions:
- (i)
xt is in ,
- (ii)
,
- (iii)
,
- (i)
-
where H > 0 is a constant K, M : [0, ∞)→[1, ∞), K is continuous, M is locally bounded and H, K, M are independent of x(·).
- (B)
The space is complete.
Definition 2 (see [31].)Hausdorff’s measure of noncompactness χY for a bounded set B in any Banach space Y is defined by χY(B) = inf{r > 0, B can be covered by finite number of balls with radii r.}
Lemma 3 (see [31].)Let Y be a Banach space and B, C ⊂ Y be bounded, then the following properties hold:
- (1)
B is precompact if and only if χY(B) = 0;
- (2)
, where and conv B are closure and convex hull of B, respectively;
- (3)
χY(B) ≤ χY(C) when B ⊂ C;
- (4)
χY(B + C) ≤ χY(B) + χY(C) where B + C = {x + y; x ∈ B, y ∈ C};
- (5)
χY(B ∪ C) = max{χY(B), χY(C)};
- (6)
χY(λB) = ∥λ∥χY(B) for any ;
- (7)
if the map Q : D(Q) ⊂ Y → Z is Lipschitz continuous with constant k then χZ(QB) ≤ kχY(B) for any bounded subset B ⊂ D(Q), where Z is a Banach space;
- (8)
if is a decreasing sequence of bounded closed nonempty subset of Y and limn→∞χY(Wn) = 0, then is nonempty and compact in Y.
Definition 4 (see [31].)The map Q : W ⊂ Y → Y is said to be a χ-contraction if there exists a positive constant k < 1 such that χYQ(C) ≤ kχY(C) for any bounded close subset C ⊂ W where Y is a Banach space.
Lemma 5 (Darbo-Sadovskii [31]). If W ⊂ Y is closed and convex and 0 ∈ W, the continuous map Q : W → W is χ-contraction, then the map Q has at least one fixed point.
Lemma 6 (see [31].) (1) If W ⊂ PC([a, b]; X) is bounded, then χ(W(t)) ≤ χPC(W) for any t ∈ [a, b] where W(t) = {u(t) : u ∈ W} ⊂ X.
(2) If W is piecewise equicontinuous on [a, b], then χ(W(t)) is piecewise continuous for t ∈ [a, b], and
(3) If W ⊂ PC([a, b]; X) is bounded and piecewise equicontinuous, then χ(W(t)) is piecewise continuous for t ∈ [a, b] and
Lemma 7 (see [35].)If the semigroup S(t) is equicontinuous and η ∈ L([0, a]; R+), then the set is equicontinuous for t ∈ [0, b].
3. Existence and Uniqueness of Mild Solution
We define mild solution of problem (1) as follows.
Definition 8. A function x : (−∞, a] → X is a mild solution of the problem (1) if x0 = ϕ; x(·)|[0,a] ∈ PC(X), , and
-
Hϕ The function t → ϕt is continuous from into and there exists a continuous bounded function such that for every .
- (Hf)
satisfies the following.
- (1)
For every and x|J ∈ PC, the function f(·, ψ) : J → X is strongly measurable for every and f(·, t) is continuous for a.e. t ∈ J.
- (2)
There exists an integrable function α : J → [0, +∞) and a monotone continuous nondecreasing function Ω : [0, +∞)→(0, +∞) such that and .
- (3)
There exists an integrable function η : J → [0, ∞) such that χ(S(s)f(t, D)) ≤ η(t)sup−∞<θ<0χ(D(θ)) for a.e. s, t, ∈J, where D(θ) = {v(θ) : v ∈ D}.
- (1)
- (Hg)
The function g(·) is continuous and g(t, ·) is Lipschitz continuous such that there exists positive constant Lg such that
() -
(HJ)
-
-
(1) There exist positive constants such that and .
-
(2) for all .
-
(1) ;
-
(2) .
-
-
(H1)
-
-
(1) ;
-
(2) .
-
Lemma 9 (see [9].)If y : (−∞, a] → X is a function such that y0 = ϕ and y|J ∈ PC(X) then
In this section y : (−∞, a] → X is the function defined by y0 = ϕ and y(t) = C(t)ϕ(0) + S(t)(z + g(0, ϕ)) on J1 = [0, t1]. Clearly where ∥y∥b = sup0≤t≤b∥y(t)∥.
Theorem 10. If the hypotheses (Hf), (Hg), (HI), (H1) are satisfied, then the initial value problem (1) has at least one mild solution.
Proof. Let S(a) be the space S(a) = {x : (−∞, a] → X∣x0 = 0, x|J ∈ PC} endowed with supremum norm ∥·∥a.
Let Γ : S(a) → S(a) be the map defined by (Γx) 0 = 0 and :
Step 1. There exists k > 0 such that Γ(Bk) ⊂ Bk, where Bk = {x ∈ S(a) : ∥x∥a ≤ k}. In fact, if we assume that the assertion is false, then for k > 0 there exist xk ∈ Bk and tk ∈ (si, ti+1] such that k < ∥Γxk(tk)∥:
Step 2. To prove that Γ is a χ-contraction. Let be split into for t > 0
So, is Lipschitz continuous with Lipschitz constant NLgaKa.
For any , W is piecewise equicontinuous since S(t) is equicontinuous. Hence from the fact that , s ∈ [0, a] and Lemma 6 and χPC(W) = sup{χ(W(t)), t ∈ J} we have
For arbitrary x1, x2 ∈ Bk and t ∈ (si, ti+1]
For arbitrary x1, x2 ∈ Bk and t ∈ (ti, si],
4. Approximate Controllability
Definition 11. A function x : (−∞, a] → X is a mild solution of the problem (24) if x0 = ϕ; x(·)|[0,a] ∈ C(J, X), the functions and g(s, xs) are integrable and the integral equation is satisfied:
Lemma 12 (see [11].)Under the assumption that h : [0, a] → X is an integrable function, such that
Set a∶ = T.
Definition 13. The set given by is the mild solution of (24)} is called reachable set of the system (24). is the reachable set of the corresponding linear control system (31).
Definition 14. The system (24) is said to be approximately controllable on [0, T] if is dense in X. The corresponding linear system is approximately controllable if is dense in X.
Lemma 15. Let X be Hilbert space and X1, X2 closed subspaces such that X = X1 + X2. Then there exists a bounded linear operator P : X → X2 such that for each x ∈ X, x = x − Px ∈ X1 and ∥x1∥ = min{∥y∥ : y ∈ X1, (1 − Q)(y) = (1 − Q)(x)} where Q denotes the orthogonal projection on X2.
- (HR)
∀ϵ > 0 and , ∃u(·) ∈ U such that .
Theorem 16. If the assumptions (Hg) and (HR) hold then the corresponding neutral system
Proof. It is sufficient to prove that since D(A) is dense in X. Let for any chosen ξ ∈ D(A), then ξ − h(T, ϕ) ∈ D(A). It can be easily seen from Lemma 12 and [28] that there exists some p ∈ C1([0, T]; X) such that
- (C1)
The function is continuous for almost all t ∈ I and F(·, z) : J → X is strongly measurable, .
- (C2)
There exists integrable functions μF, νF : I → [0, ∞) and a continuous nondecreasing function WF : [0, ∞)→(0, ∞) such that ∥F(t,z)∥2 ≤ μF(t)WF(∥z∥1) + νF(t), .
- (C3)
The function f(·) is continuous and f(t, ·) is Lipschitz continuous such that there exists positive constant Lf such that
()
Also, y : (−∞, a] → X is the function defined by y0 = ϕ and y(t) = C(t)ϕ(0) + S(t)(z + g(0, ϕ)) on J. Clearly where ∥y∥b = sup0≤t≤b∥y(t)∥.
As a continuation of coauthor Sukavanam’s work [19] and from hypothesis (B1) in [39] we assume that .
By using Lemma 15 we denote Pi the map associated to this decomposition and construct and . Also set ci = ∥Pi∥.
Lemma 17. If the hypothesis (Hϕ)–(Hg) and conditions (C1)-(C2) hold for f, g and then Γ has a fixed point.
Proof. For let and . ∀0 ≤ t ≤ a
Theorem 18. If the associated linear control system (31) is approximately controllable on J, the space and condition of the preceding Lemma 17 hold then the semilinear control system (24) with state dependent delay is approximately controllable on J.
Proof. Assume x(·) to be the mild solution and u(·) to be an admissible control function of system (31) with initial conditions x(0) = ϕ(0) and x′(0) = w + g(0, ϕ). Let z be the fixed point of Γ. So, z(0) = 0 and z(a) = Λ1(P1(F(z))) − Λ2(P2(G(z))) = 0. By Lemma 12 we can split the functions F(z), G(z) with respect to the decomposition , respectively, by setting q1 = F(z) − P1(F(z)) and q2 = G(z) − P2(G(z)). We define the function y(t) = z(t) + x(t) for t ∈ J and y0 = ϕ. So, x(a) = y(a). Thus by the properties of x and z
5. Examples
Example 1. In this section we discuss a partial differential equation applying the abstract results of this paper. In this application, is the phase space C0 × L2(h, X) (see [10]).
- (a)
the functions b(s, η, ξ), ∂b(s, η, ξ)/∂ξ are measurable, b(s, η, π) = b(s, η, 0) = 0 and
()such that . - (b)
The function is continuous and there is continuous function and .
- (c)
The functions and for all i = 1,2, …, n j = 1,2.
Hence by assumptions (a)–(c) and Theorem 10 it is ensured that mild solution to the problem (51) exists.
Example 2. Consider the second order neutral differential equation:
- (a)
The functions b(s, η, ξ), ∂b(s, η, ξ)/∂ξ are measurable, b(s, η, π) = b(s, η, 0) = 0 and
()such that . - (b)
The function is continuous and there is continuous function and .
- (c)
The functions and for all i = 1,2, …, n j = 1,2.
Here we examine the conditions (HR) for this control system. Then by using Theorem 18 we show its approximate controllability. Let for . The bounded linear operator B : L2([0, T]; U) → L2([0, T]; X) is defined by .
Hence by assumptions (a)–(c) and Theorem 18 it is ensured that the problem (51) is approximately controllable.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors would like to express sincere gratitude to the reviewer for his valuable suggestions. The first author would like to thank Ministry of Human Resource and Development with Grant no. MHR-02-23-200-429/304 for their funding.