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# 2: Ordinary differential equations

• • Contributed by William F. Trench
• Andrew G. Cowles Distinguished Professor Emeritus (Mathamatics) at Trinity University
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In order to apply mathematical methods to a physical or "real life'' problem, we must formulate the problem in mathematical terms; that is, we must construct a $$\textcolor{blue}{\mbox{mathematical model}}$$ for the problem. Many physical problems concern relationships between changing quantities. Since rates of change are represented mathematically by derivatives, mathematical models often involve equations relating an unknown function and one or more of its derivatives. Such equations are $$\textcolor{blue}{\mbox{differential equations}}$$. They are the subject of this chapter.

In this chapter, we study a particularly important class of second-order equations. Because of their many applications in science and engineering, second order differential equation has historically been the most thoroughly studied class of differential equations. Research on the theory of second order differential equations continues to the present day. This chapter is devoted to second order equations that can be written in the form

$$P_0(x)y''+P_1(x)y'+P_2(x)y=F(x).$$

Such equations are said to be $$\textcolor{blue}{\mbox{linear}}$$. As in the case of first order linear equations, $$(2.1)$$ is said to be $$\textcolor{blue}{\mbox{homogeneous}}$$ if $$F\equiv0$$, or $$\textcolor{blue}{\mbox{nonhomogeneous}}$$ if $$F\not\equiv0$$.