\begin{definition}[Definition:Bessel's Equation]
'''Bessel's equation''' is a second order ODE of the form:
:$\displaystyle x^2 \frac{\mathrm d^2 y} {\mathrm d x^2} + x \frac{\mathrm d y} {\mathrm d x} + \left({x^2 - p^2}\right) y = 0$
The parameter $p$ may be any arbitrary real or complex number.
\end{definition}