Associated Legendre function
1062015
223044640
2008-07-02T09:31:08Z
TomyDuby
2575906
/* Definition */ Added alternative notations of associated Legendre function
:''Note: This article describes a very general class of functions. An important subclass of these functions—those with integer <math>\ell</math> and ''m''—are commonly called "associated Legendre polynomials", even though they are not [[polynomial]]s when ''m'' is odd. The fully general class of functions described here, with arbitrary real or complex values of <math>\ell\,</math> and ''m'', are sometimes called "generalized Legendre functions", or just "Legendre functions". In that case the parameters are usually renamed with Greek letters.''
In [[mathematics]], the '''associated Legendre functions''' are the canonical solutions of the '''general Legendre equation'''
:<math>(1-x^2)\,y'' -2xy' + \left(\ell[\ell+1] - \frac{m^2}{1-x^2}\right)\,y = 0,\,</math>
or
:<math>([1-x^2]\,y')' + \left(\ell[\ell+1] - \frac{m^2}{1-x^2}\right)\,y = 0,\,</math>
where the indices <math>\ell</math> and ''m'' (which in general are complex quantities) are referred to as the degree and order of the associated Legendre function respectively. This equation has solutions that are nonsingular on [−1, 1] only if <math>\ell\,</math> and ''m'' are integers with 0 ≤ ''m'' ≤ <math>\ell</math>, or with trivially equivalent negative values. When in addition ''m'' is even, the function is a [[polynomial]]. When ''m'' is zero and <math>\ell\,</math> integer, these functions are identical to the [[Legendre polynomial]]s.
This [[differential equation|ordinary differential equation]] is frequently encountered in [[physics]] and other technical fields. In particular, it occurs when solving [[Laplace's equation]] (and related [[partial differential equation]]s) in [[spherical coordinates]].
==Definition==
These functions are denoted <math>P_\ell^{(m)}(x)</math>. We put the superscript in parentheses
to avoid confusing it with an exponent. Their most straightforward definition is in terms
of derivatives of ordinary [[Legendre polynomials]] (''m'' ≥ 0)
:<math>P_\ell^{(m)}(x) = (-1)^m\ (1-x^2)^{m/2}\ \frac{d^m}{dx^m}\left(P_\ell(x)\right)\,</math>
The <math>(-1)^m</math> factor in this formula is known as the [[Spherical harmonics#Condon-Shortley phase|Condon-Shortley phase]]. Some authors omit it.
Since, by [[Olinde Rodrigues|Rodrigues]]' formula,
:<math>P_\ell(x) = \frac{1}{2^\ell\,\ell!} \ \frac{d^\ell}{dx^\ell}\left([x^2-1]^\ell\right),</math>
one obtains
:<math>P_\ell^{(m)}(x) = \frac{(-1)^m}{2^\ell \ell!} (1-x^2)^{m/2}\ \frac{d^{\ell+m}}{dx^{\ell+m}}(x^2-1)^\ell.</math>
This equation allows extension of the range of ''m'' to: -''l'' ≤ ''m'' ≤ ''l''. The definitions of ''P''<sub>''l''</sub><sup>(±''m'')</sup>, resulting from this expression by substitution of ±''m'', are proportional. Indeed,
equate the coefficients of equal powers on the left and right hand side of
:<math>
\frac{d^{\ell-m}}{dx^{\ell-m}} (x^2-1)^{\ell} = c_{lm} (1-x^2)^m \frac{d^{\ell+m}}{dx^{\ell+m}}(x^2-1)^{\ell},
</math>
then it follows that the proportionality constant is
:<math>
c_{lm} = (-1)^m \frac{(\ell-m)!}{(\ell+m)!} ,
</math>
so that
:<math>
P^{(-m)}_\ell(x) = (-1)^m \frac{(\ell-m)!}{(\ell+m)!} P^{(m)}_\ell(x).
</math>
===Alternative notations===
The following notations are used in literature:
:<math>P_{\ell} ^{m}(x) = P_\ell^{(m)}(x) </math>
:<math>P_{\ell m}(x) = (-1)^m P_\ell^{(m)}(x) </math>
==Orthogonality==
Assuming <math>0 \le m \le \ell</math>, they satisfy the orthogonality condition for fixed ''m'':
:<math>\int_{-1}^{1} P_k ^{(m)} P_\ell ^{(m)} dx = \frac{2 (\ell+m)!}{(2\ell+1)(\ell-m)!}\ \delta _{k,\ell}</math>
Where <math>\delta _{k,\ell}</math> is the [[Kronecker delta]].
Also, they satisfy the orthogonality condition for fixed <math>\ell</math>:
:<math>\int_{-1}^{1} \frac{P_\ell ^{(m)} P_\ell ^{(n)}}{1-x^2}dx = \begin{cases} 0 & \mbox{if } m\neq n \\ \frac{(\ell+m)!}{m(\ell-m)!} & \mbox{if } m=n\neq0 \\ \infty & \mbox{if } m=n=0\end{cases}</math>
==Negative ''m'' and/or negative ''l''==
The differential equation is clearly invariant under a change in sign of ''m''.
The functions for negative ''m'' were shown above to be proportional to those of positive ''m'':
:<math>P_\ell ^{(-m)} = (-1)^m \frac{(\ell-m)!}{(\ell+m)!} P_\ell ^{(m)}</math>
(This followed from the Rodrigues' formula definition. This definition also makes the various recurrence formulas work for positive or negative ''m''.)
<math>\textrm{If}\quad {\mid}m{\mid} > \ell\,\quad\mathrm{then}\quad P_\ell^{(m)} = 0.\,</math>
The differential equation is also invariant under a change from <math>\ell</math> to
<math>-\ell-1</math>, and the functions for negative <math>\ell</math> are defined by
:<math>P_{-\ell} ^{(m)} = P_{\ell-1} ^{(m)}.\,</math>
==The first few associated Legendre polynomials==
The first few associated Legendre polynomials, including those for negative values of ''m'', are:
:<math>P_{0}^{0}(x)=1</math>
:<math>P_{1}^{-1}(x)=-\begin{matrix}\frac{1}{2}\end{matrix}P_{1}^{1}(x)</math>
:<math>P_{1}^{0}(x)=x</math>
:<math>P_{1}^{1}(x)=-(1-x^2)^{1/2}</math>
:<math>P_{2}^{-2}(x)=\begin{matrix}\frac{1}{24}\end{matrix}P_{2}^{2}(x)</math>
:<math>P_{2}^{-1}(x)=-\begin{matrix}\frac{1}{6}\end{matrix}P_{2}^{1}(x)</math>
:<math>P_{2}^{0}(x)=\begin{matrix}\frac{1}{2}\end{matrix}(3x^{2}-1)</math>
:<math>P_{2}^{1}(x)=-3x(1-x^2)^{1/2}</math>
:<math>P_{2}^{2}(x)=3(1-x^2)</math>
:<math>P_{3}^{-3}(x)=-\begin{matrix}\frac{1}{720}\end{matrix}P_{3}^{3}(x)</math>
:<math>P_{3}^{-2}(x)=\begin{matrix}\frac{1}{120}\end{matrix}P_{3}^{2}(x)</math>
:<math>P_{3}^{-1}(x)=-\begin{matrix}\frac{1}{12}\end{matrix}P_{3}^{1}(x)</math>
:<math>P_{3}^{0}(x)=\begin{matrix}\frac{1}{2}\end{matrix}(5x^3-3x)</math>
:<math>P_{3}^{1}(x)=-\begin{matrix}\frac{3}{2}\end{matrix}(5x^{2}-1)(1-x^2)^{1/2}</math>
:<math>P_{3}^{2}(x)=15x(1-x^2)</math>
:<math>P_{3}^{3}(x)=-15(1-x^2)^{3/2}</math>
:<math>P_{4}^{-4}(x)=\begin{matrix}\frac{1}{40320}\end{matrix}P_{4}^{4}(x)</math>
:<math>P_{4}^{-3}(x)=-\begin{matrix}\frac{1}{5040}\end{matrix}P_{4}^{3}(x)</math>
:<math>P_{4}^{-2}(x)=\begin{matrix}\frac{1}{360}\end{matrix}P_{4}^{2}(x)</math>
:<math>P_{4}^{-1}(x)=-\begin{matrix}\frac{1}{20}\end{matrix}P_{4}^{1}(x)</math>
:<math>P_{4}^{0}(x)=\begin{matrix}\frac{1}{8}\end{matrix}(35x^{4}-30x^{2}+3)</math>
:<math>P_{4}^{1}(x)=-\begin{matrix}\frac{5}{2}\end{matrix}(7x^3-3x)(1-x^2)^{1/2}</math>
:<math>P_{4}^{2}(x)=\begin{matrix}\frac{15}{2}\end{matrix}(7x^2-1)(1-x^2)</math>
:<math>P_{4}^{3}(x)= - 105x(1-x^2)^{3/2}</math>
:<math>P_{4}^{4}(x)=105(1-x^2)^{2}</math>
<!--
:<math>P_{5}^{-5}(x)={1\over 3840}\left(\sqrt{1-x^2}\right)^{5}</math>
:<math>P_{5}^{-4}(x)={1\over 384}\left(\sqrt{1-x^2}\right)^{4}x</math>
:<math>P_{5}^{-3}(x)={1\over 384}\left(\sqrt{1-x^2}\right)^{3}(9x^{2}-1)</math>
:<math>P_{5}^{-2}(x)={1\over 16}\left(\sqrt{1-x^2}\right)^{2}(3x^{3}-1x)</math>
:<math>P_{5}^{-1}(x)={1\over 16}\left(\sqrt{1-x^2}\right)(21x^{4}-14x^{2}+1)</math>
:<math>P_{5}^{0}(x)={1\over 8}(63x^{5}-70x^{3}+15x)</math>
:<math>P_{5}^{1}(x)={-15\over 8}\left(\sqrt{1-x^2}\right)(21x^{4}-14x^{2}+1)</math>
:<math>P_{5}^{2}(x)={105\over 2}\left(\sqrt{1-x^2}\right)^{2}(3x^{3}-1x)</math>
:<math>P_{5}^{3}(x)={-105\over 2}\left(\sqrt{1-x^2}\right)^{3}(9x^{2}-1)</math>
:<math>P_{5}^{4}(x)=945\left(\sqrt{1-x^2}\right)^{4}x</math>
:<math>P_{5}^{5}(x)=-945\left(\sqrt{1-x^2}\right)^{5}</math>
:<math>P_{6}^{-6}(x)={1\over 46080}\left(\sqrt{1-x^2}\right)^{6}</math>
:<math>P_{6}^{-5}(x)={1\over 3840}\left(\sqrt{1-x^2}\right)^{5}x</math>
:<math>P_{6}^{-4}(x)={1\over 3840}\left(\sqrt{1-x^2}\right)^{4}(11x^{2}-1)</math>
:<math>P_{6}^{-3}(x)={1\over 384}\left(\sqrt{1-x^2}\right)^{3}(11x^{3}-3x)</math>
:<math>P_{6}^{-2}(x)={1\over 128}\left(\sqrt{1-x^2}\right)^{2}(33x^{4}-18x^{2}+1)</math>
:<math>P_{6}^{-1}(x)={1\over 16}\left(\sqrt{1-x^2}\right)(33x^{5}-30x^{3}+5x)</math>
:<math>P_{6}^{0}(x)={1\over 16}(231x^{6}-315x^{4}+105x^{2}-5)</math>
:<math>P_{6}^{1}(x)={-21\over 8}\left(\sqrt{1-x^2}\right)(33x^{5}-30x^{3}+5x)</math>
:<math>P_{6}^{2}(x)={105\over 8}\left(\sqrt{1-x^2}\right)^{2}(33x^{4}-18x^{2}+1)</math>
:<math>P_{6}^{3}(x)={-315\over 2}\left(\sqrt{1-x^2}\right)^{3}(11x^{3}-3x)</math>
:<math>P_{6}^{4}(x)={945\over 2}\left(\sqrt{1-x^2}\right)^{4}(11x^{2}-1)</math>
:<math>P_{6}^{5}(x)=-10395\left(\sqrt{1-x^2}\right)^{5}x</math>
:<math>P_{6}^{6}(x)=10395\left(\sqrt{1-x^2}\right)^{6}</math>
:<math>P_{7}^{-7}(x)={1\over 645120}\left(\sqrt{1-x^2}\right)^{7}</math>
:<math>P_{7}^{-6}(x)={1\over 46080}\left(\sqrt{1-x^2}\right)^{6}x</math>
:<math>P_{7}^{-5}(x)={1\over 46080}\left(\sqrt{1-x^2}\right)^{5}(13x^{2}-1)</math>
:<math>P_{7}^{-4}(x)={1\over 3840}\left(\sqrt{1-x^2}\right)^{4}(13x^{3}-3x)</math>
:<math>P_{7}^{-3}(x)={1\over 3840}\left(\sqrt{1-x^2}\right)^{3}(143x^{4}-66x^{2}+3)</math>
:<math>P_{7}^{-2}(x)={1\over 384}\left(\sqrt{1-x^2}\right)^{2}(143x^{5}-110x^{3}+15x)</math>
:<math>P_{7}^{-1}(x)={1\over 128}\left(\sqrt{1-x^2}\right)(429x^{6}-495x^{4}+135x^{2}-5)</math>
:<math>P_{7}^{0}(x)={1\over 16}(429x^{7}-693x^{5}+315x^{3}-35x)</math>
:<math>P_{7}^{1}(x)={-7\over 16}\left(\sqrt{1-x^2}\right)(429x^{6}-495x^{4}+135x^{2}-5)</math>
:<math>P_{7}^{2}(x)={63\over 8}\left(\sqrt{1-x^2}\right)^{2}(143x^{5}-110x^{3}+15x)</math>
:<math>P_{7}^{3}(x)={-315\over 8}\left(\sqrt{1-x^2}\right)^{3}(143x^{4}-66x^{2}+3)</math>
:<math>P_{7}^{4}(x)={3465\over 2}\left(\sqrt{1-x^2}\right)^{4}(13x^{3}-3x)</math>
:<math>P_{7}^{5}(x)={-10395\over 2}\left(\sqrt{1-x^2}\right)^{5}(13x^{2}-1)</math>
:<math>P_{7}^{6}(x)=135135\left(\sqrt{1-x^2}\right)^{6}x</math>
:<math>P_{7}^{7}(x)=-135135\left(\sqrt{1-x^2}\right)^{7}</math>
:<math>P_{8}^{-8}(x)={1\over 10321920}\left(\sqrt{1-x^2}\right)^{8}</math>
:<math>P_{8}^{-7}(x)={1\over 645120}\left(\sqrt{1-x^2}\right)^{7}x</math>
:<math>P_{8}^{-6}(x)={1\over 645120}\left(\sqrt{1-x^2}\right)^{6}(15x^{2}-1)</math>
:<math>P_{8}^{-5}(x)={1\over 15360}\left(\sqrt{1-x^2}\right)^{5}(5x^{3}-1x)</math>
:<math>P_{8}^{-4}(x)={1\over 15360}\left(\sqrt{1-x^2}\right)^{4}(65x^{4}-26x^{2}+1)</math>
:<math>P_{8}^{-3}(x)={1\over 768}\left(\sqrt{1-x^2}\right)^{3}(39x^{5}-26x^{3}+3x)</math>
:<math>P_{8}^{-2}(x)={1\over 256}\left(\sqrt{1-x^2}\right)^{2}(143x^{6}-143x^{4}+33x^{2}-1)</math>
:<math>P_{8}^{-1}(x)={1\over 128}\left(\sqrt{1-x^2}\right)(715x^{7}-1001x^{5}+385x^{3}-35x)</math>
:<math>P_{8}^{0}(x)={1\over 128}(6435x^{8}-12012x^{6}+6930x^{4}-1260x^{2}+35)</math>
:<math>P_{8}^{1}(x)={-9\over 16}\left(\sqrt{1-x^2}\right)(715x^{7}-1001x^{5}+385x^{3}-35x)</math>
:<math>P_{8}^{2}(x)={315\over 16}\left(\sqrt{1-x^2}\right)^{2}(143x^{6}-143x^{4}+33x^{2}-1)</math>
:<math>P_{8}^{3}(x)={-3465\over 8}\left(\sqrt{1-x^2}\right)^{3}(39x^{5}-26x^{3}+3x)</math>
:<math>P_{8}^{4}(x)={10395\over 8}\left(\sqrt{1-x^2}\right)^{4}(65x^{4}-26x^{2}+1)</math>
:<math>P_{8}^{5}(x)={-135135\over 2}\left(\sqrt{1-x^2}\right)^{5}(5x^{3}-1x)</math>
:<math>P_{8}^{6}(x)={135135\over 2}\left(\sqrt{1-x^2}\right)^{6}(15x^{2}-1)</math>
:<math>P_{8}^{7}(x)=-2027025\left(\sqrt{1-x^2}\right)^{7}x</math>
:<math>P_{8}^{8}(x)=2027025\left(\sqrt{1-x^2}\right)^{8}</math>
:<math>P_{9}^{-9}(x)={1\over 185794560}\left(\sqrt{1-x^2}\right)^{9}</math>
:<math>P_{9}^{-8}(x)={1\over 10321920}\left(\sqrt{1-x^2}\right)^{8}x</math>
:<math>P_{9}^{-7}(x)={1\over 10321920}\left(\sqrt{1-x^2}\right)^{7}(17x^{2}-1)</math>
:<math>P_{9}^{-6}(x)={1\over 645120}\left(\sqrt{1-x^2}\right)^{6}(17x^{3}-3x)</math>
:<math>P_{9}^{-5}(x)={1\over 215040}\left(\sqrt{1-x^2}\right)^{5}(85x^{4}-30x^{2}+1)</math>
:<math>P_{9}^{-4}(x)={1\over 3072}\left(\sqrt{1-x^2}\right)^{4}(17x^{5}-10x^{3}+1x)</math>
:<math>P_{9}^{-3}(x)={1\over 3072}\left(\sqrt{1-x^2}\right)^{3}(221x^{6}-195x^{4}+39x^{2}-1)</math>
:<math>P_{9}^{-2}(x)={1\over 256}\left(\sqrt{1-x^2}\right)^{2}(221x^{7}-273x^{5}+91x^{3}-7x)</math>
:<math>P_{9}^{-1}(x)={1\over 256}\left(\sqrt{1-x^2}\right)(2431x^{8}-4004x^{6}+2002x^{4}-308x^{2}+7)</math>
:<math>P_{9}^{0}(x)={1\over 128}(12155x^{9}-25740x^{7}+18018x^{5}-4620x^{3}+315x)</math>
:<math>P_{9}^{1}(x)={-45\over 128}\left(\sqrt{1-x^2}\right)(2431x^{8}-4004x^{6}+2002x^{4}-308x^{2}+7)</math>
:<math>P_{9}^{2}(x)={495\over 16}\left(\sqrt{1-x^2}\right)^{2}(221x^{7}-273x^{5}+91x^{3}-7x)</math>
:<math>P_{9}^{3}(x)={-3465\over 16}\left(\sqrt{1-x^2}\right)^{3}(221x^{6}-195x^{4}+39x^{2}-1)</math>
:<math>P_{9}^{4}(x)={135135\over 8}\left(\sqrt{1-x^2}\right)^{4}(17x^{5}-10x^{3}+1x)</math>
:<math>P_{9}^{5}(x)={-135135\over 8}\left(\sqrt{1-x^2}\right)^{5}(85x^{4}-30x^{2}+1)</math>
:<math>P_{9}^{6}(x)={675675\over 2}\left(\sqrt{1-x^2}\right)^{6}(17x^{3}-3x)</math>
:<math>P_{9}^{7}(x)={-2027025\over 2}\left(\sqrt{1-x^2}\right)^{7}(17x^{2}-1)</math>
:<math>P_{9}^{8}(x)=34459425\left(\sqrt{1-x^2}\right)^{8}x</math>
:<math>P_{9}^{9}(x)=-34459425\left(\sqrt{1-x^2}\right)^{9}</math>
:<math>P_{10}^{-10}(x)={1\over 3715891200}\left(\sqrt{1-x^2}\right)^{10}</math>
:<math>P_{10}^{-9}(x)={1\over 185794560}\left(\sqrt{1-x^2}\right)^{9}x</math>
:<math>P_{10}^{-8}(x)={1\over 185794560}\left(\sqrt{1-x^2}\right)^{8}(19x^{2}-1)</math>
:<math>P_{10}^{-7}(x)={1\over 10321920}\left(\sqrt{1-x^2}\right)^{7}(19x^{3}-3x)</math>
:<math>P_{10}^{-6}(x)={1\over 10321920}\left(\sqrt{1-x^2}\right)^{6}(323x^{4}-102x^{2}+3)</math>
:<math>P_{10}^{-5}(x)={1\over 645120}\left(\sqrt{1-x^2}\right)^{5}(323x^{5}-170x^{3}+15x)</math>
:<math>P_{10}^{-4}(x)={1\over 43008}\left(\sqrt{1-x^2}\right)^{4}(323x^{6}-255x^{4}+45x^{2}-1)</math>
:<math>P_{10}^{-3}(x)={1\over 3072}\left(\sqrt{1-x^2}\right)^{3}(323x^{7}-357x^{5}+105x^{3}-7x)</math>
:<math>P_{10}^{-2}(x)={1\over 3072}\left(\sqrt{1-x^2}\right)^{2}(4199x^{8}-6188x^{6}+2730x^{4}-364x^{2}+7)</math>
:<math>P_{10}^{-1}(x)={1\over 256}\left(\sqrt{1-x^2}\right)(4199x^{9}-7956x^{7}+4914x^{5}-1092x^{3}+63x)</math>
:<math>P_{10}^{0}(x)={1\over 256}(46189x^{10}-109395x^{8}+90090x^{6}-30030x^{4}+3465x^{2}-63)</math>
:<math>P_{10}^{1}(x)={-55\over 128}\left(\sqrt{1-x^2}\right)(4199x^{9}-7956x^{7}+4914x^{5}-1092x^{3}+63x)</math>
:<math>P_{10}^{2}(x)={495\over 128}\left(\sqrt{1-x^2}\right)^{2}(4199x^{8}-6188x^{6}+2730x^{4}-364x^{2}+7)</math>
:<math>P_{10}^{3}(x)={-6435\over 16}\left(\sqrt{1-x^2}\right)^{3}(323x^{7}-357x^{5}+105x^{3}-7x)</math>
:<math>P_{10}^{4}(x)={45045\over 16}\left(\sqrt{1-x^2}\right)^{4}(323x^{6}-255x^{4}+45x^{2}-1)</math>
:<math>P_{10}^{5}(x)={-135135\over 8}\left(\sqrt{1-x^2}\right)^{5}(323x^{5}-170x^{3}+15x)</math>
:<math>P_{10}^{6}(x)={675675\over 8}\left(\sqrt{1-x^2}\right)^{6}(323x^{4}-102x^{2}+3)</math>
:<math>P_{10}^{7}(x)={-11486475\over 2}\left(\sqrt{1-x^2}\right)^{7}(19x^{3}-3x)</math>
:<math>P_{10}^{8}(x)={34459425\over 2}\left(\sqrt{1-x^2}\right)^{8}(19x^{2}-1)</math>
:<math>P_{10}^{9}(x)=-654729075\left(\sqrt{1-x^2}\right)^{9}x</math>
:<math>P_{10}^{10}(x)=654729075\left(\sqrt{1-x^2}\right)^{10}</math>
-->
==Recurrence formula==
These functions have a number of recurrence properties:
:<math>(\ell-m+1)P_{\ell+1}^{(m)}(x) = (2\ell+1)xP_{\ell}^{(m)}(x) - (\ell+m)P_{\ell-1}^{(m)}(x)</math>
:<math>P_{\ell+1}^{(m)}(x) = P_{\ell-1}^{(m)}(x) - (2\ell+1)\sqrt{1-x^2}P_{\ell}^{(m-1)}(x)</math>
:<math>\sqrt{1-x^2}P_{\ell}^{(m+1)}(x) = (\ell-m)xP_{\ell}^{(m)}(x) - (\ell+m)P_{\ell-1}^{(m)}(x)</math>
:<math>(x^2-1){P_{\ell}^{(m)}}'(x) = {\ell}xP_{\ell}^{(m)}(x) - (\ell+m)P_{\ell-1}^{(m)}(x)</math>
:<math>(x^2-1){P_{\ell}^{(m)}}'(x) = -(\ell+m)(\ell-m+1)\sqrt{1-x^2}P_{\ell}^{(m-1)}(x) - mxP_{\ell}^{(m)}(x)</math>
Helpfull identities (initial values for the first recursion):
:<math>P_{\ell}^{(\ell)}(x) = (-1)^l (2\ell-1)!! (1- x^2)^{(l/2)}</math>
:<math>P_{\ell +1}^{(\ell)}(x) = x (2\ell+1) P_{\ell}^{(\ell)}</math>
with !! the [[Factorial#Double_factorial|double factorial]].
==Gaunt's formula==<!-- This section is linked from [[Gaunt's formula]]. See [[WP:MOS#Section management]] -->
The integral over the product of three associated Legendre polynomials (with orders matching as shown below)
turns out to be necessary when doing atomic calculations of the [[Hartree-Fock]] variety where matrix elements of
the Coulomb operator are needed. For this we have Gaunt's formula
<ref>From John C. Slater ''Quantum Theory of Atomic Structure'', McGraw-Hill (New York, 1960), Volume I, page 309, which cites the original work of J. A. Gaunt, ''Philosophical Transactions of the Royal Society of London'', A228:151 (1929)</ref>
{| border="0" cellpadding="0" cellspacing="0"
|-
|<math>\frac{1}{2} \int_{-1}^1 dx P_l^u(x) P_m^v(x) P_n^w(x) = </math>
|<math>(-1)^{s-m-w}\frac{(m+v)!(n+w)!(2s-2n)!s!}{(m-v)!(s-l)!(s-m)!(s-n)!(2s+1)!}</math>
|-
|
|<math>\times \ \sum_{t=p}^q (-1)^t \frac{(l+u+t)!(m+n-u-t)!}{t!(l-u-t)!(m-n+u+t)!(n-w-t)!}</math>
|}
This formula is to be used under the following assumptions:
# the degrees are non-negative integers <math>l,m,n\ge0</math>
# all three orders are non-negative integers <math>u,v,w\ge 0</math>
# <math>u</math> is the largest of the three orders
# the orders sum up <math>u=v+w</math>
# the degrees obey <math> m\ge n</math>
Other quantities appearing in the formula are defined as
: <math> 2s = l+m+n </math>
: <math> p = max(0,n-w-u) </math>
: <math> q = min(m+n-u,l-u,n-w) </math>
The integral is zero unless
# the sum of degrees is even so that <math>s</math> is an integer
# the triangular condition is satisfied <math>m+n\ge l \ge m-n</math>
==The Legendre functions, and the hypergeometric function==
These functions may be defined for general complex parameters and argument:
:<math>P_{\lambda}^{(\mu)}(z) = \frac{1}{\Gamma(1-\mu)} \left[\frac{1+z}{1-z}\right]^{\mu/2} \,_2F_1 (-\lambda, \lambda+1; 1-\mu; \frac{1-z}{2})</math>
where <math>\Gamma</math> is the [[gamma function]] and <math>\,_2F_1</math> is the [[hypergeometric function]]
:<math>\,_2F_1 (\alpha, \beta; \gamma; z) = \frac{\Gamma(\gamma)}{\Gamma(\alpha)\Gamma(\beta)} \sum_{n=0}^\infty\frac{\Gamma(n+\alpha)\Gamma(n+\beta)}{\Gamma(n+\gamma)\ n!}z^n,</math>
so that
:<math>P_{\lambda}^{(\mu)}(z) = \frac{1}{\Gamma(-\lambda)\Gamma(\lambda+1)} \left[\frac{1+z}{1-z}\right]^{\mu/2} \sum_{n=0}^\infty\frac{\Gamma(n-\lambda)\Gamma(n+\lambda+1)}{\Gamma(n+1-\mu)\ n!}\left(\frac{1-z}{2}\right)^n.</math>
They are called the '''Legendre functions''' when defined in this more general way. They satisfy
the same differential equation as before:
:<math>(1-z^2)\,y'' -2zy' + \left(\lambda[\lambda+1] - \frac{\mu^2}{1-z^2}\right)\,y = 0.\,</math>
Since this is a second order differential equation, it has a second solution,
<math>Q_\lambda^{(\mu)}(z)</math>, defined as:
:<math>Q_{\lambda}^{(\mu)}(z) = \frac{\sqrt{\pi}\ \Gamma(\lambda+\mu+1)}{2^{\lambda+1}\Gamma(\lambda+3/2)}\frac{1}{z^{\lambda+\mu+1}}(1-z^2)^{\mu/2} \,_2F_1 \left(\frac{\lambda+\mu+1}{2}, \frac{\lambda+\mu+2}{2}; \lambda+\frac{3}{2}; \frac{1}{z^2}\right)</math>
<math>P_\lambda^{(\mu)}(z)</math> and <math>Q_\lambda^{(\mu)}(z)</math> both obey the various
recurrence formulas given previously.
==Reparameterization in terms of angles==
These functions are most useful when the argument is reparameterized in terms of angles,
letting <math>x = \cos\theta</math>:
:<math>P_\ell^{(m)}(\cos\theta) = (-1)^m (\sin \theta)^m\ \frac{d^m}{d(\cos\theta)^m}\left(P_\ell(\cos\theta)\right)\,</math>
The first few polynomials, parameterized this way, are:
:<math>P_{0}^{0}(\cos\theta)=1</math>
:<math>P_{1}^{0}(\cos\theta)=\cos\theta</math>
:<math>P_{1}^{1}(\cos\theta)=-\sin\theta</math>
:<math>P_{2}^{0}(\cos\theta)=\begin{matrix}\frac{1}{2}\end{matrix}(3\cos^2\theta-1)</math>
:<math>P_{2}^{1}(\cos\theta)=-3\cos\theta\sin\theta</math>
:<math>P_{2}^{2}(\cos\theta)=3\sin^2\theta</math>
:<math>P_{3}^{0}(\cos\theta)=\begin{matrix}\frac{1}{2}\end{matrix}(5\cos^3\theta-3\cos\theta)</math>
:<math>P_{3}^{1}(\cos\theta)=-\begin{matrix}\frac{3}{2}\end{matrix}(5\cos^2\theta-1)\sin\theta</math>
:<math>P_{3}^{2}(\cos\theta)=15\cos\theta\sin^2\theta</math>
:<math>P_{3}^{3}(\cos\theta)=-15\sin^3\theta</math>
:<math>P_{4}^{0}(\cos\theta)=\begin{matrix}\frac{1}{8}\end{matrix}(35\cos^4\theta-30\cos\theta^{2}+3)</math>
:<math>P_{4}^{1}(\cos\theta)=-\begin{matrix}\frac{5}{2}\end{matrix}(7\cos^3\theta-3\cos\theta)\sin\theta</math>
:<math>P_{4}^{2}(\cos\theta)=\begin{matrix}\frac{15}{2}\end{matrix}(7\cos^2\theta-1)\sin^2\theta</math>
:<math>P_{4}^{3}(\cos\theta)=-105\cos\theta\sin^3\theta</math>
:<math>P_{4}^{4}(\cos\theta)=105\sin^4\theta</math>
For fixed ''m'', <math>P_\ell^{(m)}(\cos\theta)</math> are orthogonal, parameterized by θ over <math>[0, \pi]</math>, with weight <math>\sin \theta</math>:
:<math>\int_{0}^{\pi} P_k^{(m)}(\cos\theta) P_\ell^{(m)}(\cos\theta)\,\sin\theta\,d\theta = \frac{2 (\ell+m)!}{(2\ell+1)(\ell-m)!}\ \delta _{k,\ell}</math>
Also, for fixed <math>\ell</math>:
:<math>\int_{0}^{\pi}P_\ell^{(m)}(\cos\theta) P_\ell^{(n)}(\cos\theta) \csc\theta\,d\theta = \begin{cases} 0 & \mbox{if } m\neq n \\ \frac{(\ell+m)!}{m(\ell-m)!} & \mbox{if } m=n\neq0 \\ \infty & \mbox{if } m=n=0\end{cases}</math>
In terms of θ, <math>P_\ell^{(m)}(\cos \theta)</math> are solutions of
:<math>\frac{d^{2}y}{d\theta^2} + \cot \theta \frac{dy}{d\theta} + \left[\lambda - \frac{m^2}{\sin^2\theta}\right]\,y = 0\,</math>
More precisely, given an integer ''m''<math>\ge</math>0, the above equation has
nonsingular solutions only when <math>\lambda = \ell(\ell+1)\,</math> for <math>\ell</math>
an integer<math>{\ge}m</math>, and those solutions are proportional to
<math>P_\ell^{(m)}(\cos \theta)</math>.
==Applications in physics: Spherical harmonics==
{{main|Spherical harmonics}}
In many occasions in [[physics]], associated Legendre polynomials in terms of angles occur where [[spherical]] [[symmetry]] is involved. The colatitude angle in [[spherical coordinates]] is
the angle <math>\theta</math> used above. The longitude angle, <math>\phi</math>, appears in a multiplying factor. Together, they make a set of functions called [[spherical harmonic]]s.
These functions express the symmetry of the [[Riemann sphere|two-sphere]] under the action of the [[Lie group]] SO(3). As such, Legendre polynomials can be generalized to express the symmetries of semi-simple Lie groups and [[Riemannian symmetric space]]s.
What makes these functions useful is that they are central to the solution of the equation
<math>\nabla^2\psi + \lambda\psi = 0</math> on the surface of a sphere. In spherical coordinates θ (colatitude) and φ (longitude), the [[Laplacian]] is
:<math>\nabla^2\psi = \frac{\partial^2\psi}{\partial\theta^2} + \cot \theta \frac{\partial \psi}{\partial \theta} + \csc^2 \theta\frac{\partial^2\psi}{\partial\phi^2}.</math>
When the [[partial differential equation]]
:<math>\frac{\partial^2\psi}{\partial\theta^2} + \cot \theta \frac{\partial \psi}{\partial \theta} + \csc^2 \theta\frac{\partial^2\psi}{\partial\phi^2} + \lambda \psi = 0</math>
is solved by the method of [[separation of variables]], one gets a φ-dependent part <math>\sin(m\phi)</math> or <math>\cos(m\phi)</math> for integer m≥0, and an equation for the θ-dependent part
:<math>\frac{d^{2}y}{d\theta^2} + \cot \theta \frac{dy}{d\theta} + \left[\lambda - \frac{m^2}{\sin^2\theta}\right]\,y = 0\,</math>
for which the solutions are <math>P_\ell^{(m)}(\cos \theta)</math> with <math>\ell{\ge}m</math>
and <math>\lambda = \ell(\ell+1)</math>.
Therefore, the equation
:<math>\nabla^2\psi + \lambda\psi = 0</math>
has nonsingular separated solutions only when <math>\lambda = \ell(\ell+1)</math>,
and those solutions are proportional to
:<math>P_\ell^{(m)}(\cos \theta)\ \cos (m\phi)\ \ \ \ 0 \le m \le \ell</math>
and
:<math>P_\ell^{(m)}(\cos \theta)\ \sin (m\phi)\ \ \ \ 0 < m \le \ell.</math>
For each choice of <math>\ell</math>, there are <math>2\ell+1</math> functions
for the various values of ''m'' and choices of sine and cosine.
They are all orthogonal in both <math>\ell</math> and ''m'' when integrated over the
surface of the sphere.
The solutions are usually written in terms of [[complex exponential]]s:
:<math>Y_{\ell, m}(\theta, \phi) = \sqrt{\frac{(2\ell+1)(\ell-m)!}{4\pi(\ell+m)!}}\ P_\ell^{(m)}(\cos \theta)\ e^{im\phi}\qquad -\ell \le m \le \ell.
</math>
The functions <math>Y_{\ell, m}(\theta, \phi)</math> are the [[spherical harmonics]], and the quantity in the square root is a normalizing factor.
Recalling the relation between the associated Legendre functions of positive and negative ''m'', it is easily shown that the spherical harmonics satisfy the identity<ref>This identity can also be shown by relating the spherical harmonics to [[Wigner D-matrix|Wigner D-matrices]] and use of the time-reversal property of the latter. The relation between associated Legendre functions of ±''m'' can then be proved from the complex conjugation identity of the spherical harmonics.</ref>
:<math>Y_{\ell, m}^*(\theta, \phi) = (-1)^m Y_{\ell, -m}(\theta, \phi).</math>
The spherical harmonic functions form a complete orthonormal set of functions in the sense of [[Fourier series]]. It should be noted that workers in the fields of geodesy, geomagnetism and spectral analysis use a different phase and normalization factor than given here (see [[spherical harmonics]]).
When a 3-dimensional spherically symmetric partial differential equation is solved by the method of separation of variables in spherical coordinates, the part that remains after removal of the radial part is typically
of the form <math>\nabla^2\psi(\theta, \phi) + \lambda\psi(\theta, \phi) = 0</math>, and hence the solutions are spherical harmonics.
==See also==
* [[Angular momentum]]
* [[Gaussian quadrature]]
* [[Legendre polynomials]]
* [[Spherical harmonic]]s
* [[Whipple's transformation of Legendre functions]]
==External links==
*[http://mathworld.wolfram.com/LegendrePolynomial.html Legendre polynomials in MathWorld]
==Notes ==
<references/>
==References==
* Arfken G.B., Weber H.J., ''Mathematical methods for physicists'', (2001) Academic Press, ISBN 0-12-059825-6 ''See Section 12.5''. (Uses a different sign convention.)
* A.R. Edmonds, ''Angular Momentum in Quantum Mechanics'', (1957) Princeton University Press, ISBN 0-691-07912-9 ''See chapter 2''.
* E. U. Condon and G. H. Shortley, ''The Theory of Atomic Spectra'', (1970) Cambridge, England: The University Press. {{Oclc number|5388084}} ''See chapter 3''
* {{Abramowitz_Stegun_ref|8|332}}
* F. B. Hildebrand, ''Advanced Calculus for Applications'', (1976) Prentice Hall, ISBN 0-13-011189-9
* Belousov, S. L. (1962), ''Tables of normalized associated Legendre polynomials'', Mathematical tables series Vol. 18, Pergamon Press, 379p.
[[Category:Atomic physics]]
[[Category:Orthogonal polynomials]]
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