# Math Help - Simplification of Spherical Bessel functions

1. ## Simplification of Spherical Bessel functions

From here:

Why isn't the $(-x)^n (1/x)^n$ product simplified to $(-1)^n$?

Thanks in advance for any help.

2. Originally Posted by MrWaeseL
From here:

Why isn't the $(-x)^n (1/x)^n$ product simplified to $(-1)^n$?

Thanks in advance for any help.
Because it's an operator not just an algebraic expression. To demonstrate let $n = 2$ (and $f(x) = \frac{\sin x}{x}$ ) so

$
(-x)^2 \left(\frac{1}{x} \frac{d}{dx} \right)^2 f(x) = x^2 \frac{1}{x} \frac{d}{dx} \left(\frac{1}{x} \frac{df}{dx} \right) = \frac{d^2f}{d x^2} - \frac{1}{x} \frac{df}{dx}.
$

3. I see...so it would be wrong to state that $\left(\frac{1}{x} \frac{d}{dx} \right)^n = (\frac{1}{x})^n (\frac{d}{dx})^n$?

4. Originally Posted by MrWaeseL
I see...so it would be wrong to state that $\left(\frac{1}{x} \frac{d}{dx} \right)^n = (\frac{1}{x})^n (\frac{d}{dx})^n$?
Absolutely. It only applies when $n = 1$.