# Legendre functions in number theory

I have heard that Legendre functions are important in number theory. Can any one tell me how?

The Legendre function of the first kind $P_s$ is defined by \begin{eqnarray*}P_s(x) =& \frac{1}{2\pi}\int_{-\pi}^{\pi}\left(x+\sqrt{x^2-1}\cos\theta\right)^s d\theta
\newline =&\frac{1}{\pi}\int_0^1\left(x+\sqrt{x^2-1}(2t-1)\right)^s\frac{dt}{\sqrt{t(1-t)}},& s\in\mathbb{C}, x\ge 1. \end{eqnarray*}

• I've never been fond of that integral representation TBH; I had always preferred the generating function or Rodrigues or even the hypergeometric definition. – J. M. is a poor mathematician Sep 20 '10 at 12:06
• @J. M. Sure, but, Rodrigues formula is for Legendre polynomials and Hypergeometric representation is more adequate on a disc. We can also define $P_s$ as the solution to $\frac{d}{dx}\left((x^2-1)u'(x)\right) = s(s+1)u$, such that $u(1)=1$. – AD. Sep 20 '10 at 12:22
• Well, you can take derivatives to arbitrary order, remember? ;) Also ${}_2 F_1$ admits an analytic continuation... anyway this is way off what you want; I'll look with interest at what the experts may come up with. – J. M. is a poor mathematician Sep 20 '10 at 13:25
• Are you sure there is no mixup between Legendre functions and the Legendre symbol? – Accipitridae Sep 20 '10 at 19:17
• @Accipitridae No it was a connection between Legendre functions and prime numbers.. (maybe it was associated Legendre function, that I don't remember..) – AD. Sep 20 '10 at 20:53

Perhaps this has something for you: http://wis.kuleuven.be/analyse/walter/coimbra.pdf

This uses the Legendre polynomials in proving irrationality of certain numbers.

If you accept enumerative combinatorics as part of number theory, here is a nice application of the Legendre polynomials.

Count the number of ways to insert paired, grammatical and non-repeating parentheses enclosing at least two letters in a word of length $n > 1$. Define $a_1 = 1$ and ignore outmost paratheses. For $n = 2$, there is only one way $(ab) = ab$. For $n = 3$, we have $abc, (ab)c, a(bc)$. For $n = 4$, we have $abcd$, $(ab)cd$, $a(bc)d$, $ab(cd)$, $(ab)(cd)$, $a(bcd)$, $a(b(cd))$, $a((bc)d)$, $(abc)d$, $(a(bc))d$, and $((ab)c)d$. And, so on.

The sequence continues {$1$, $1$, $3$, $11$, $45$, $197$, $903$, $4279 \dots$ } (A001003) and the terms are called Super Catalan numbers or Little Schroeder numbers. In general, \begin{eqnarray} a_n = \tfrac{1}{n} \sum_{k =0}^{n-2} \binom{2n-k-2}{n-1} \binom{n-2}{k} = \tfrac{1}{4n}(3P_{n-1}(3) - P_{n-2}(3)) + \tfrac{1}{2} \epsilon(n), \end{eqnarray} where $\epsilon$ is the unit arithmetic function, which is $1$ when the argument is $1$ and $0$ otherwise.

In fact, there are at least $10$ different combinatorial problems enumerated by this sequence, involving restricted lattice paths, integer compositions and even matrix row-sums. The Online Encyclopedia of Integer Sequences has the present state of knowledge about these fascinating numbers; see there reference section of http://oeis.org/A001003 for relevant papers and books.

• In the formula, what is the "unit arithmetic function" epsilon(n)? – T.. Nov 15 '10 at 18:08