#### Abstract

We give an existence and uniqueness theorem for solutions of inhomogeneous impulsive boundary value problem (BVP) for planar Hamiltonian systems. Green's function that is needed for representing the solutions is obtained and its properties are listed. The uniqueness of solutions is connected to a Lyapunov type inequality for the corresponding homogeneous BVP.

#### 1. Introduction

The planar Hamiltonian system of 2-linear first-order equations has the form where is a symmetric matrix with piecewise continuous real-valued entries, and is the so called symplectic identity. Setting the Hamiltonian system can be rewritten in a more convenient way as

Our aim in this work is to prove an existence and uniqueness theorem for solutions of the related BVP for inhomogeneous Hamiltonian system under impulse effect of the formwhere(i),,,, andare real sequences forwith (ii) , whereand is continuous on each interval, the limitsexist andfor(iii)forandfor;andare given real numbers.

We also setandfor convenience.

By a solution of the impulsive BVP (6a)–(6c), we mean nontrivial functionssuch thatsatisfies system (6a)–(6c) for all.

The corresponding homogeneous BVP takes the form

Note that if we take then we obtain as a special case of (6a), (6b), and (6c) the impulsive BVP for second-order differential equations of the form

To the best of our knowledge although many results have been obtained for linear impulsive boundary value problems by using different techniques, there is little known for the linearHamiltonian systems under impulse effect.

The existence and uniqueness of linear impulsive boundary value problem for the first-order equations are considered in [1–4]. For the second-order case we refer to [5, 6] in which the integral representation of the solution of second order linear impulsive boundary value problems is given by using Green’s function and the existence and uniqueness of the solutions are obtained. Variational technique approach for the existence of the solutions of linear and nonlinear impulsive boundary value problems can be found in [7–10]. In [11], the method of upper and lower solutions is employed for the existence of solutions of nonlinear impulsive boundary value problems. For a detailed discussion on boundary value problems for higher-order linear impulsive equations we refer to [12]. Basic theory of impulsive differential equations is contained in the seminal book [13].

Our method of proof is based on Green’s function formulation and Lyapunov type inequalities for linear Hamiltonian system under impulse effect. There are many studies on Lyapunov type inequalities and their applications for linear ordinary differential equations [14] and for systems [15–17] as well as for linear impulsive differential equations and systems [18, 19]. However, the use of a Lyapunov type inequality in connection with BVPs seems to be limited.

#### 2. Preliminaries

##### 2.1. Lyapunov Type Inequality for Homogeneous Problem

In this section we provide a Lyapunov type inequality to be used for the uniqueness of the inhomogeneous BVP. The obtained inequality is sharper than the one given by the present authors in [20] in the sense thatis replaced by.

Theorem 1. *If the homogeneous BVP (8a), (8b), and (8c) has a real solutionsuch thaton, then one has the Lyapunov type inequality:
**
whereand.*

*Proof. *Define
forand, where we put again,and make a convention thatif.

It is not difficult to see from (8a), (8b), (8c), and (12) that

Since we assumed that,is continuous on. Moreover,,, andfor all. We may assume without loss of generality thaton.

Using (13) and (14) we obtain
Integrating (16) fromtoand using (15) and (17) lead to
from which we have

On the other hand, from the first equation in (13), we have
Let
If we integrate (20) fromto, we see that
and so
Using the obvious estimate
and then applying Cauchy-Schwarz inequality, we have
Similarly, by integrating (21) fromtoand following the above procedure, we get

Now we recall the elementary inequality:
forand. In view of (26) and (27) setting
we have
Combining (19) and (30) results in
Finally, sincefor, from (31) we obtain the desired inequality:

##### 2.2. Green’s Function

Here we derive Green’s function to be used for the representation of the solutions of the inhomogeneous BVP.

Let be a fundamental matrix for (8a), (8b) and set

Define the rectangles and the triangles

Green’s function (pair) and its properties are given in the next theorem.

Theorem 2. *Suppose that the homogeneous BVP (8a)–(8c) has only the trivial solution. Let
**
Note that the inverse of matrixexists in view of the assumption (see also the proof of Theorem 4).**Then the pair of functions
**
constitutes Green’s function for (6a), (6b), and (6c). Moreover, we have the following properties: *(G1)*is continuous and bounded on,*(G2)*is continuous and bounded on the rectangleswithand on the trianglesand,*(G3)*satisfies the following jump conditions:(a) where(b),(c),*(G4)

*, considered as a function of, is left continuous and satisfies*

*whereis any of the intervalsor*(G5)(G6)

*, considered as a function of, is left continuous and satisfies (39).*

*Proof. *(G1) and (G2) are trivial. Let us consider (G3)(a) follows from
To see (b), we write for,
For (c), let; then

Next, we consider (G4). By definition, it is easy to see thatis left continuous function at. Let us consider the interval. The later is similar. The first equation in (39) is direct consequences of (c) and the definition of. Clearly,

The proofs of (G5) and (G6) are similar to (a) and (G4), respectively.

*Remark 3. *One can easily rewrite Green’s function (pair) in terms of the solutions of system (8a), (8b). Indeed,
and since
we may write

#### 3. The Main Result

Our main result is the following theorem.

Theorem 4. *Let (i)–(iii) hold. If
**
then BVP (6a), (6b), and (6c) has a unique solution. Moreover,is expressible as
**
where
**
and Green’s function pairis given by (38).*

*Proof. *We first prove the uniqueness. It suffices to show that the homogeneous BVP (8a)–(8c) has only the trivial solution. Leton. By Theorem 1, we see that Lyapunov type inequality (11) holds contradicting the inequality (47). Thusfor all. Moreover, by (6a), (6b), and (6c) we have
which results infor. Taking limit we see that. As a result we obtainfor all. This completes the uniqueness of the solutions.

For the existence, we start with the variation of parameters formula and write the general solution of system (6a), (6b) as
Clearly, the boundary condition is satisfied if
where.

Since we have the uniqueness of solutions, the matrixmust have an inverse. Setting
we may solvefrom (52) uniquely:
Hence,
Therefore the unique solution of the BVP (6a)–(6c) can be expressed as

Let us now consider the BVP (10a), (10b), (10c), and (10d). In this case it is not difficult to see that the corresponding Green’s function (pair) becomes whereis the first row of the (Wronskian) matrix:

Corollary 5. *Suppose thatandare piece-wise continuous on,, andfor.If
**
then the BVP (10a), (10b), (10c), and (10d) has a unique solutionwhich is expressible as
**
where
**
and Green’s function pairis given by (57).*

*Remark 6. *The results in this work are new even if the impulses are absent. The statements of the corresponding theorems are left to the reader.