cpp/numeric/special functions/sph legendre

@1-3@ Computes the of degree, order , and polar angle. @A@ Additional overloads are provided for all integer types, which are treated as.

Return value
If no errors occur, returns the value of the spherical associated Legendre function (that is, spherical harmonic with ϕ = 0) of, , and , where the spherical harmonic function is defined as $Ym l(theta,ϕ) = (-1)m []1/2 Pm l(cos(theta))eimϕ$ where $Pm l(x)$ is ) and $|m|≤l$.

Note that the Condon-Shortley phase term $(-1)m$ is included in this definition because it is omitted from the definition of $Pm l$ in std.

Error handling
Errors may be reported as specified in math_errhandling.
 * If the argument is NaN, NaN is returned and domain error is not reported.
 * If $l≥128$, the behavior is implementation-defined.