return sqrt((2*l+1)(l-m)!/(4*pi*(l+m)!)), the normalization
coefficient for spherical harmonic Ylm with respect to the
associated Legendre function Plm. In this implementation,
0<=m<=l; use symmetry for m<0, or use sines and cosines
instead of complex exponentials. Unlike Plm, array L and M
arguments are permissible here.
WARNING: These get combinitorially small with large L and M;
probably Plm is simultaneously blowing up and should be
normalized directly in legndr if what you want is Ylm. But
I don't feel like working all that out -- if you need large
L and M results, you should probably be working with some
sort of asymptotic form anyway...
interpreted function, defined at i/legndr.i line 55