# Integral involving Legendre polynomials

i want to evaluate the following integral:

$$a_n = \frac{2n+1}{2}\int_{-1}^1 \frac{P_n(x)}{\sqrt{2-2x}} \text{dx} \ \text{where P_n(x) is the n^{th} Legendre Polynomial. }$$ I am expecting the result to be $1$. I tried to set it up on MAPLE and tried a large number of $n$'s ( 1,2 ,3,1000,2000) and the result is $1$. However, i am looking for a rigorous proof. I looked into the table of integrals involving legendre polynomials and could not find a case that suits mine.

The generating function for Legendre polynomials is $$\frac{1}{\sqrt{1-2xt+t^2}}=\sum_{n}P_n(x)t^n$$
Just use $t=1$ in here to expand your integrand in Legendre polynomials and then use the orthogonality condition.
• Can you provide me the reference where we are allowed to replace $t$ by 1. All the references state that $|t|$ < 1. – outlawoutlawz Oct 11 '17 at 16:25
• @Marcel: Probably because you do not state on what subsets that equality is valid (i.e. what are the allowed values for $x$ and $t$ in it). In particular, how does one know whether the power series in $t$ converges in $t=1$? What happens for $t=x=1$? – Alex M. Nov 4 '17 at 16:21
• @Marcel: First, I'm not the downvoter. Second, even as an answer to the question it shouldn't ignore studying the matters of convergence that arise (and I see two of them: the convergence of the series and the convergence of some improper integrals at $1$). – Alex M. Nov 4 '17 at 19:28