Existence and multiplicity of solutions for some second-order systems on time scales with impulsive effects
© Zhou et al.; licensee Springer 2012
Received: 17 September 2012
Accepted: 6 December 2012
Published: 21 December 2012
In this paper, we present a recent approach via variational methods and critical point theory to obtain the existence of solutions for the nonautonomous second-order system on time scales with impulsive effects
where , (), , is a symmetric matrix-valued function defined on with for all , (, ) are continuous and . Finally, two examples are presented to illustrate the feasibility and effectiveness of our results.
- (A)is Δ-measurable in t for every and continuously differentiable in x for Δ-a.e. , and there exist , such that
for all and Δ-a.e. , where denotes the gradient of in x.
For the sake of convenience, in the sequel, we denote , .
In , the authors study the Sobolev’s spaces on time scales and their properties. As applications, they present a recent approach via variational methods and the critical point theory to obtain the existence of solutions for (1.2).
When , , and is not a zero matrix, until now the variational structure of (1.1) has not been studied.
In , the authors establish some sufficient conditions on the existence of solutions of (1.3) by means of some critical point theorems when . When , until now, it is unknown whether problem (1.1) has a variational structure or not.
Impulsive effects exist widely in many evolution processes in which their states are changed abruptly at certain moments of time. The theory of impulsive differential systems has been developed by numerous mathematicians (see [3–5]). Applications of impulsive differential equations with or without delays occur in biology, medicine, mechanics, engineering, chaos theory and so on (see [6–9]).
For a second-order differential equation , one usually considers impulses in the position u and the velocity . However, in the motion of spacecraft, one has to consider instantaneous impulses depending on the position that result in jump discontinuities in velocity, but with no change in position (see ). The impulses only on the velocity occur also in impulsive mechanics (see ). An impulsive problem with impulses in the derivative only is considered in .
The study of dynamical systems on time scales is now an active area of research. One of the reasons for this is the fact that the study on time scales unifies the study of both discrete and continuous processes, besides many others. The pioneering works in this direction are Refs. [13–17]. The theory of time scales was initiated by Stefan Hilger in his Ph.D. thesis in 1988, providing a rich theory that unifies and extends discrete and continuous analysis [18, 19]. The time scales calculus has a tremendous potential for applications in some mathematical models of real processes and phenomena studied in physics, chemical technology, population dynamics, biotechnology and economics, neural networks and social sciences (see ). For example, it can model insect populations that are continuous while in season (and may follow a difference scheme with variable step-size), die out in winter, while their eggs are incubating or dormant, and then hatch in a new season, giving rise to a nonoverlapping population.
There have been many approaches to study solutions of differential equations on time scales, such as the method of lower and upper solutions, fixed-point theory, coincidence degree theory and so on (see [1, 20–29]). In , authors used the fixed point theorem of strict-set-contraction to study the existence of positive periodic solutions for functional differential equations with impulse effects on time scales. However, the study of the existence and multiplicity of solutions for differential equations on time scales using the variational method has received considerably less attention (see, for example, [1, 29]). The variational method is, to the best of our knowledge, novel and it may open a new approach to deal with nonlinear problems, with some type of discontinuities such as impulses.
Motivated by the above, we research the existence of variational construction for problem (1.1) in an appropriate space of functions and study the existence of solutions for (1.1) by some critical point theorems in this paper. All these results are new.
2 Preliminaries and statements
In this section, we present some fundamental definitions and results from the calculus on time scales and Sobolev’s spaces on time scales that will be required below. These are a generalization to of definitions and results found in .
Definition 2.1 ([, Definition 1.1])
(supplemented by and , where ∅ denotes the empty set). A point is called right-scattered, left-scattered, if , hold, respectively. Points that are right-scattered and left-scattered at the same time are called isolated. Also, if and , then t is called right-dense, and if and , then t is called left-dense. Points that are right-dense and left-dense at the same time are called dense. The set which is derived from the time scale as follows. If has a left-scattered maximum m, then ; otherwise, .
respectively. Note that if b is left-dense and if b is left-scattered. We denote , therefore if b is left-dense and if b is left-scattered.
Definition 2.2 ([, Definition 1.10])
We call the delta (or Hilger) derivative of f at t. The function f is delta (or Hilger) differentiable on provided exists for all . The function is then called the delta derivative of f on .
Definition 2.3 ([, Definition 2.3])
and let . Then we define (provided it exists). We call the delta (or Hilger) derivative of f at t. The function f is delta (or Hilger) differentiable provided exists for all . The function is then called the delta derivative of f on .
Definition 2.4 ([, Definition 2.7])
For a function , we will talk about the second derivative provided is differentiable on with derivative .
Definition 2.5 ([, Definition 2.5])
For a function , we will talk about the second derivative provided is differentiable on with derivative .
The Δ-measure and Δ-integration are defined as those in .
Definition 2.6 ([, Definition 2.7])
Assume that is a function, and let A be a Δ-measurable subset of . f is integrable on A if and only if () are integrable on A, and .
Definition 2.7 ([, Definition 2.3])
Let . B is called a Δ-null set if . Say that a property P holds Δ-almost everywhere (Δ-a.e.) on B, or for Δ-almost all (Δ-a.a.) if there is a Δ-null set such that P holds for all .
We have the following theorem.
Theorem 2.1 ([, Theorem 2.1])
where denotes the inner product in .
Definition 2.8 ([, Definition 2.11])
A function . We say that f is absolutely continuous on (i.e., ) if for every , there exists such that if is a finite pairwise disjoint family of subintervals of satisfying , then .
Now, we recall the Sobolev space on defined in . For the sake of convenience, in the sequel we let .
Definition 2.9 ([, Definition 2.12])
is true for every with . These two sets are, as a class of functions, equivalent. It is the characterization of functions in in terms of functions in too. That is the following theorem.
Theorem 2.2 ([, Theorem 2.5])
By identifying with its absolutely continuous representative for which (2.2) holds, the set can be endowed with the structure of a Banach space. That is the following theorem.
Theorem 2.3 ([, Theorem 2.21])
The Banach space has some important properties.
Theorem 2.4 ([, Theorem 2.23])
holds for all , where .
Theorem 2.5 ([, Theorem 2.25])
If the sequence converges weakly to u in , then converges strongly in to u.
Theorem 2.6 ([, Theorem 2.27])
3 Variational setting
In this section, we recall some basic facts which will be used in the proofs of our main results. In order to apply the critical point theory, we make a variational structure. From this variational structure, we can reduce the problem of finding solutions of (1.1) to the one of seeking the critical points of a corresponding functional.
If , by identifying with its absolutely continuous representative for which (2.2) holds, then u is absolutely continuous and . In this case, may not hold for some . This leads to impulsive effects.
Considering the above, we introduce the following concept solution for problem (1.1).
holds for any .
for all .
for all . Thus, φ is continuously differentiable on and (3.3) holds. □
By Definition 3.1 and Lemma 3.1, the weak solutions of problem (1.1) correspond to the critical points of φ.
Remark 3.1 K has only finitely many eigenvalues with since K is compact on . Hence is finite dimensional. Notice that is a compact perturbation of the self-adjoint operator I. By a well-known theorem, we know that 0 is not in the essential spectrum of . Hence, is a finite dimensional space too.
To prove our main results, we need the following definitions and theorems.
Definition 3.2 ([, ])
Let X be a real Banach space and . I is said to be satisfying (PS) condition on X if any sequence for which is bounded and as , possesses a convergent subsequence in X.
Firstly, we state the local linking theorem.
For every multi-index , we denote by the space . We say , . A sequence is admissible if, for every , there is such that .
Definition 3.3 ([, Definition 2.2])
contains a subsequence which converges to a critical point of I.
Theorem 3.1 [, Theorem 2.2]
Suppose that satisfies the following assumptions:
(I2) I satisfies condition.
(I3) I maps bounded sets into bounded sets.
(I4) For every , as , .
Then I has at least two critical points.
Remark 3.2 Since , by the condition (I1) of Theorem 3.1, 0 is the critical point of I. Thus, under the conditions of Theorem 3.1, I has at least one nontrivial critical point.
Secondly, we state another three critical point theorems.
Theorem 3.2 ([, Theorem 5.29])
Let E be a Hilbert space with and . Suppose , satisfies (PS) condition, and
(I5) , where and is bounded and self-adjoint, ,
(I6) is compact, and
Q is bounded and ,
S and ∂Q link.
Then I possesses a critical value .
Theorem 3.3 ([, Theorem 9.12])
Let E be a Banach space. Let be an even functional which satisfies the (PS) condition and . If , where V is finite dimensional, and I satisfies
(I8) there are constants such that , where ,
(I9) for each finite dimensional subspace , there is an such that on ,
then I possesses an unbounded sequence of critical values.
We denote by the topology on E induced by a semi-norm family , and let w and denote the weak-topology and weak*-topology, respectively.
For a functional , we write . Recall that is said to be weak sequentially continuous if, for any in E, one has for each , i.e., is sequentially continuous. For , we say that Φ satisfies the condition if any sequence such that and as contains a convergent subsequence.
() for any , is -closed, and is continuous;
Theorem 3.4 ()
Assume that Φ is even and ()-() are satisfied. Then Φ has at least pairs of critical points with critical values less than or equal to provided Φ satisfies the condition for all .
Remark 3.3 In our applications, we take = so that is the product topology on given by the weak topology on X and the strong topology on Y.
4 Main results
Lemma 4.1 is compact on .
The continuity of and this imply that in . The proof is complete. □
First of all, we give two existence results.
Theorem 4.1 Suppose that (A) and the following conditions are satisfied.
(F1) uniformly for Δ-a.e. ,
(F2) uniformly for Δ-a.e. ,
(F6) for every , , ,
Then problem (1.1) has at least two weak solutions. The one is a nontrivial weak solution, the other is a trivial weak solution.
We divide our proof into four parts in order to show Theorem 4.1.
Firstly, we show that φ satisfies the condition.
This implies , and hence . Therefore, in . Hence φ satisfies the condition.
Secondly, we show that φ maps bounded sets into bounded sets.
for all . Thus, φ maps bounded sets into bounded sets.
Thirdly, we claim that φ has a local linking at 0 with respect to .
Let . Then φ satisfies the condition of Theorem 3.1.
where . Hence, for every , as and .
Thus, by Theorem 3.1, problem (1.1) has at least one nontrivial weak solution. The proof is complete. □
is the solution of problem (4.22).
Theorem 4.2 Assume that (A), (F5), (F6), (F7) and the following conditions are satisfied.
(F8) uniformly for Δ-a.e. ,
(F9) there exist constants and such that for all and ,
(F10) for all and Δ-a.e. .
Then problem (1.1) has at least one nontrivial weak solution.
Proof Set , and . Then E is a real Hilbert space, , and .
for large k. Since , , by (4.26), is bounded in .
Moreover, we can prove that is compact (see [, p.1437]). It follows from (3.4), (4.29) and Lemma 4.1 that φ satisfies the conditions (I5), (I6) and (I7)(i) with of Theorem 3.2.
for large due to .
for large .
Summing up the above, φ satisfies all conditions of Theorem 3.2. Hence, φ possesses a critical value , and hence problem (1.1) has at least one nontrivial weak solution. The proof is complete. □
Remark 4.1 There are a number of functions satisfying (A), (F8), (F9) and (F10), for example, .
Next, we given two multiplicity results.
Theorem 4.3 Assume that (A), (F5), (F7), (F8), (F9) and the following conditions are satisfied.
(F11) (, ) are odd.
(F12) is even in x and .
Then problem (1.1) has an unbounded sequence of weak solutions.
This implies that there is an such that on .
Moreover, by (F10) and (F12), we know that φ is even and . In view of Theorem 3.3, φ has a sequence of critical points such that . If is bounded in E, then by the definition of φ, one knows that is also bounded, a contradiction. Hence, is unbounded in E. The proof is completed. □
for all , . All conditions of Theorem 4.3 hold. According to Theorem 4.3, problem (4.31) has an unbounded sequence of weak solutions.
Remark 4.2 In Theorem 4.3, if we delete the condition ‘’, we have the following theorem.
Theorem 4.4 Assume that (A), (F5), (F7), (F8), (F9), (F11) and the following condition are satisfied.
(F13) is even in x.
Then problem (1.1) has an infinite sequence of distinct weak solutions.
Proof Set , and in Theorem 3.4. Then, from the proof of Theorem 4.3, we know that , , φ is even, satisfies the (PS) condition, and there are constants such that and , where .
Consequently, for each finite dimensional subspace , the condition () holds. Moreover, by and , we know that () holds too. Therefore, the conclusion follows from Theorem 2.6. □
This work is supported by the National Natural Sciences Foundation of People’s Republic of China under Grant 10971183, the Natural Sciences Foundation of Yunnan Province (2011Y116, 2012FB111, IRTSTYN) and the third batch young skeleton teachers training plan of Yunnan University (XT412003).
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