Multinomial coefficient l! / (lx! ly! lz!) of one angular triple.
This is the combinatorial prefactor C(l) of paper eq. 4, used when
contracting pairs of Cartesian angular features into rotational
invariants.
- Parameters:
lxlylz (sequence of int) – The (lx, ly, lz) triple.
- Returns:
(lx+ly+lz)! / (lx! ly! lz!).
- Return type:
int
