cpp/experimental/special functions/legendre

@1@ Computes the unassociated Legendre polynomials of the degree and argument @4@ A set of overloads or a function template accepting an argument of any integral type. Equivalent to after casting the argument to.

Return value
If no errors occur, value of the order- unassociated Legendre polynomial of, that is $(x2 -1)n$, is returned.

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
 * The function is not required to be defined for $|x|>1$
 * If is greater or equal than 128, the behavior is implementation-defined