[Math] Spherical Bessel Functions

bessel functionsderivatives

So I have been given a formula for the spherical Bessel functions in the form of $$ j_\ell(x)=(-x)^\ell \left(\frac{1}{x}\frac{d}{dx}\right)^\ell\frac{\sin(x)}{x} $$ which is Rayleigh's formula. I've been asked to show this satisfies Helmholtz's equation, however, I don't know how to diffentiate the middle part (containing the differential operator). To differentiate this, do I use the product rule as I normally would taking the $-x $, $\frac{1}{x}\frac{d}{dx}$ and $\frac{\sin(x)}{x}$ separately and pulling down the powers?

Many thanks in advance.

Best Answer

Quoting Wikipedia, when solving the Helmholtz equation in spherical coordinates by separation of variables, the radial equation has the form: $$ x^2 y'' + 2x y' + \left[x^2-n(n+1)\right] y = 0. \tag{1}$$ For $n=0$, it is straightforward to check that $y_0=\operatorname{sinc}(x)$ is a solution.

Suppose now that a solution of $(1)$ has the form $y=(-x)^n f(x)$. Then $f(x)$ satisfies the ODE:

$$ x\, f + 2(1+n)\,f' +x\, f'' = 0.\tag{2} $$ Can you prove now that a solution of $(2)$ is given by $\left(\frac{1}{x}\frac{d}{dx}\right)^n y_0$?