Prelude to Series Solutions of Linear Second Order Equations
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In this Chapter, we study a class of second order differential equations that occur in many applications, but cannot be solved in closed form in terms of elementary functions. Here are some examples:
These equations and others considered in this chapter can be written in the form
P0(x)y″+P1(x)y′+P2(x)y=0,
where P0, P1, and P2 are polynomials with no common factor. For most equations that occur in applications, these polynomials are of degree two or less. We’ll impose this restriction, although the methods that we’ll develop can be extended to the case where the coefficient functions are polynomials of arbitrary degree, or even power series that converge in some circle around the origin in the complex plane.
Since Equation ??? does not in general have closed form solutions, we seek series representations for solutions. We’ll see that if P0(0)≠0 then solutions of (A) can be written as power series y=∞∑n=0anxn that converge in an open interval centered at x=0.