- Research Article
- Open Access
On Connection between Second-Order Delay Differential Equations and Integrodifferential Equations with Delay
Advances in Difference Equations volume 2010, Article number: 143298 (2009)
The existence and uniqueness of solutions and a representation of solution formulas are studied for the following initial value problem: , , , , . Such problems are obtained by transforming second-order delay differential equations to first-order differential equations.
1. Introduction and Preliminaries
The second order delay differential equation
attracts the attention of many mathematicians because of their significance in applications.
In particular, Minorsky  in 1962 considered the problem of stabilizing the rolling of a ship by an "activated tanks method" in which ballast water is pumped from one position to another. To solve this problem, he constructed several delay differential equations with damping described by (1.1).
Despite the obvious importance in applications, there are only few papers on delay differential equations with damping.
One of the methods used to study (1.1) is transforming the second-order delay differential equation to a first-order differential or integrodifferential equations with delay. A transformation of the type
If are Lebesgue measurable and locally essentially bounded, are Lebesgue measurable functions, , if , , there exists a locally absolutely continuous function such that the inequality
is valid for all sufficiently large , and the equation
has a nonoscillatory solution, then (1.1) has a nonoscillatory solution, too.
Now we will briefly describe the scheme of another transformation, different from the one used in  (in this explanation we omit exact assumptions related to the functions used, which are formulated later).
Consider an auxiliary equation
with the initial condition
where is the fundamental matrix of (1.5) and if .
If we denote , then (1.1) can be rewritten in the form
Applying (1.7) and equality to (1.8), we have the following equation
Then (1.1) is transformed into the integrodifferential equation with delay
Since (1.11) is a result of transforming (1.1), qualitative properties of (1.11) such as the existence and uniqueness of solutions, oscillation and nonoscillation, stability and asymptotic behavior can imply similar qualitative properties of (1.1).
The advantage of the suggested method in comparison with the method used in  is that a second order delay equation is reduced to one first-order integrodifferential delay equation while in  a second-order equation is reduced to a system of a nonlinear inequality and a linear delay equation.
This paper aims to investigate the problems of the existence, uniqueness and solution representation of (1.11). Problems related to oscillation/nonoscillation, stability and applications to second-order equations will be studied in our forthcoming papers. Throughout this paper, will denote the matrix or vector norm used.
2. Main Results
Together with (1.11) we consider an initial condition
We will assume that the following conditions hold:
(a1)For all , the elements , of the matrix function are measurable in the square , the elements , of the vector function are measurable in the interval ,
(a2) is a measurable scalar function satisfying , .
(a3)The initial function is a Borel bounded function.
A function is called a solution of the problem (1.11), (2.1) if it is a locally absolutely continuous function on , satisfies equation (1.11) on almost everywhere, and initial conditions (2.1) for .
Let conditions (a1)–(a3) hold. Then there exists a unique solution of problem (1.11), (2.1).
It is sufficient to prove that there exists a unique solution of (1.11), (2.1) on the interval for any .
Then , and (1.11), (2.1) takes the form
Denote the characteristic function of the interval . We will assume that if . Since
Hence problem (2.4) can be transformed into
Finally, problem (2.4) has the form
where . Consider the linear integral operator
in the space of all Lebesgue integrable functions with the norm .
is a unique solution of (1.11), (2.1).
Let be the zero matrix and the identity matrix.
For each , the solution of the problem
is called the fundamental matrix of (1.11). (Here is the partial derivative of with respect to its first argument.)
Let conditions (a1)–(a3) hold. Then the unique solution of (1.11), (2.1) can be represented in the form
for where is defined by (2.5).
In the proof we will use notation defined in the proof of Theorem 2.1. The existence and uniqueness of a solution of (1.11), (2.1) is a consequence of Theorem 2.1. Thus, we will only prove the solution representation formula (2.16). Problem (1.11), (2.1) is equivalent to (2.4). We need to show that the function
where is the fundamental matrix of (1.11) is the solution of problem (2.4). For convenience, we will write instead of assuming that , if .
Equality (2.17) implies
We consider the left-hand side of (2.4) if assuming to have the form (2.17). With the help of the last relation, we have
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Leonid Berezansky was partially supported by grant 25/5 "Systematic support of international academic staff at Faculty of Electrical Engineering and Communication, Brno University of Technology" (Ministry of Education, Youth and Sports of the Czech Republic) and by grant 201/07/0145 of the Czech Grant Agency (Prague). Josef Diblík was supported by grant 201/08/0469 of the Czech Grant Agency (Prague), and by the Council of Czech Government grant MSM 00216 30503 and MSM 00216 30519. Zdeněk Šmarda was supported by the Council of Czech Government grant MSM 00216 30503 and MSM 00216 30529.