# $2 . \mathrm{II} . 6 \mathrm{D} \quad$

Define the Wronskian $W(x)$ associated with solutions of the equation

$\frac{d^{2} y}{d x^{2}}+p(x) \frac{d y}{d x}+q(x) y=0$

and show that

$W(x) \propto \exp \left(-\int^{x} p(\xi) d \xi\right) .$

Evaluate the expression on the right when $p(x)=-2 / x$.

Given that $p(x)=-2 / x$ and that $q(x)=-1$, show that solutions in the form of power series,

$y=x^{\lambda} \sum_{n=0}^{\infty} a_{n} x^{n} \quad\left(a_{0} \neq 0\right)$

can be found if and only if $\lambda=0$ or 3 . By constructing and solving the appropriate recurrence relations, find the coefficients $a_{n}$ for each power series.

You may assume that the equation is satisfied by $y=\cosh x-x \sinh x$ and by $y=\sinh x-x \cosh x$. Verify that these two solutions agree with the two power series found previously, and that they give the $W(x)$ found previously, up to multiplicative constants.

[Hint: $\left.\cosh x=1+\frac{x^{2}}{2 !}+\frac{x^{4}}{4 !}+\ldots, \quad \sinh x=x+\frac{x^{3}}{3 !}+\frac{x^{5}}{5 !}+\ldots .\right]$