Solution (source code)

= Solution

Here $L_3=-i\hbar(x_1\partial_{x_2}-x_2\partial_{x_1})$. The radial factor and $x_3^n$ are annihilated by the angular <derivative>, while
$$
L_3(x_1+ix_2)=\hbar(x_1+ix_2).
$$
The product rule therefore gives
$$
L_3\chi_{m,n}=m\hbar\chi_{m,n}.
$$
Replacing $i$ by $-i$ gives <eigenvalue> $-m\hbar$ for $(x_1-ix_2)^mx_3^nf(r)$.