4.II .31 A. 31 \mathrm{~A}

Asymptotic Methods
Part II, 2008

The Bessel equation of order nn is

z2y+zy+(z2n2)y=0.z^{2} y^{\prime \prime}+z y^{\prime}+\left(z^{2}-n^{2}\right) y=0 .

Here, nn is taken to be an integer, with n0n \geqslant 0. The transformation w(z)=z12y(z)w(z)=z^{\frac{1}{2}} y(z) converts (1) to the form

w+q(z)w=0w^{\prime \prime}+q(z) w=0

where

q(z)=1(n214)z2q(z)=1-\frac{\left(n^{2}-\frac{1}{4}\right)}{z^{2}}

Find two linearly independent solutions of the form

w=eszk=0ckzρkw=e^{s z} \sum_{k=0}^{\infty} c_{k} z^{\rho-k}

where ckc_{k} are constants, with c00c_{0} \neq 0, and ss and ρ\rho are to be determined. Find recurrence relationships for the ckc_{k}.

Find the first two terms of two linearly independent Liouville-Green solutions of (2) for w(z)w(z) valid in a neighbourhood of z=z=\infty. Relate these solutions to those of the form (3).