Function: pollegendre
Section: polynomials
C-Name: pollegendre_eval0
Prototype: LDGD0,L,
Help: pollegendre(n,{a='x},{flag=0}): legendre polynomial of degree n evaluated
 at a. If flag is 1, return [P_{n-1}(a), P_n(a)].
Description:
  (small,?var):gen    pollegendre($1,$2)
  (small,gen):gen     pollegendre_eval($1,$2)
Doc: $n^{\text{th}}$ \idx{Legendre polynomial} $P_{n}$ evaluated at $a$
 (\kbd{'x} by default), where
 $$P_{n}(x) = \dfrac{1}{2^{n} n!} \dfrac{d^{n}}{dx^{n}}(x^{2}-1)^{n}\;.$$
 If \fl\ is 1, return $[P_{n-1}(a), P_{n}(a)]$.
Variant: To obtain the $n$-th Legendre polynomial $P_{n}$ in variable $v$,
 use \fun{GEN}{pollegendre}{long n, long v}. To obtain $P_{n}(a)$,
 use \fun{GEN}{pollegendre_eval}{long n, GEN a}.
