Here is the whole problem, I understand the context... but I never encountered this notation before. It looks like set notation but it is not defining a set of real numbers.
Problem: Writing the radial wave function, $R(r)=\frac{u(r]}{r}$, show that the radial equation becomes: \begin{align} -\frac{\hbar^2}{2m}\frac{d^2 u}{dr^2} + \left[ V(r) + \frac{\hbar^2 \ell (\ell + 1)}{2mr^2}\right]u = Eu \end{align} Compare this with the one dimensional Schrodinger equation. What's similar, what's different?
Question: How do I start this problem? Specifically, what is the notation in the prompt?