cln_mirror/doc/polynomial/legendre.tex

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\begin{document}

The Legendre polynomials  \( P_{n}(x) \) are defined through 
\[
P_{n}(x)=\frac{1}{2^{n}n!}\cdot \left( \frac{d}{dx}\right) ^{n}(x^{2}-1)^{n}\]
(For a motivation
of the  \( 2^{n} \) in the denominator, look at  \( P_{n}(x) \) modulo an odd prime  \( p \), and
observe that  \( P_{n}(x)\equiv P_{p-1-n}(x)\mod p \) for  \( 0\leq n\leq p-1 \). This wouldn't hold if the  \( 2^{n} \) factor in the denominator
weren't present.)

\begin{description}

\item [Theorem:]~

\end{description}

 \( P_{n}(x) \) satisfies the recurrence relation


\[
P_{0}(x)=1\]



\[
(n+1)\cdot P_{n+1}(x)=(2n+1)x\cdot P_{n}(x)-n\cdot P_{n-1}(x)\]
for  \( n\geq 0 \) and the differential equation  \( (1-x^{2})\cdot P_{n}^{''}(x)-2x\cdot P_{n}^{'}(x)+(n^{2}+n)\cdot P_{n}(x)=0 \) for all  \( n\geq 0 \).

\begin{description}

\item [Proof:]~

\end{description}

Let  \( F:=\sum ^{\infty }_{n=0}P_{n}(x)\cdot z^{n} \) be the generating function of the sequence of polynomials. It
is the diagonal series of the power series
\[
G:=\sum _{m,n=0}^{\infty }\frac{1}{2^{n}m!}\cdot \left( \frac{d}{dx}\right) ^{m}(x^{2}-1)^{n}\cdot y^{m}\cdot z^{n}\]
Because the Taylor series
development theorem holds in formal power series rings (see [1], section
2.16), we can simplify
\begin{eqnarray*}
G & = & \sum _{n=0}^{\infty }\frac{1}{2^{n}}\cdot \left( \sum _{m=0}^{\infty }\frac{1}{m!}\cdot \left( \frac{d}{dx}\right) ^{m}(x^{2}-1)^{n}\cdot y^{m}\right) \cdot z^{n}\\
 & = & \sum _{n=0}^{\infty }\frac{1}{2^{n}}\cdot \left( (x+y)^{2}-1\right) ^{n}\cdot z^{n}\\
 & = & \frac{1}{1-\frac{1}{2}\left( (x+y)^{2}-1\right) z}
\end{eqnarray*}
We take over the terminology from the ``diag\_rational''
paper; here  \( R=Q[x] \) and  \( M=Q[[x]] \) (or, if you like it better,  \( M=H(C) \), the algebra of
functions holomorphic in the entire complex plane).  \( G\in M[[y,z]] \) is rational;
hence  \( F \) is algebraic over  \( R[z] \). Let's proceed exactly as in the ``diag\_series''
paper.  \( F(z^{2}) \) is the coefficient of  \( t^{0} \) in
\[
G(zt,\frac{z}{t})=\frac{2t}{2t-\left( (x+zt)^{2}-1\right) z}=\frac{2t}{-z^{3}\cdot t^{2}+2(1-xz^{2})\cdot t+(z-x^{2}z)}\]
The splitting field of the denominator
is  \( L=Q(x)(z)(\alpha ) \) where 
\[
\alpha _{1/2}=\frac{1-xz^{2}\pm \sqrt{1-2xz^{2}+z^{4}}}{z^{3}}\]

\[
\alpha =\alpha _{1}=\frac{2}{z^{3}}-\frac{2x}{z}+\frac{1-x^{2}}{2}z+\cdots \in Q(x)[[z]][\frac{1}{z}]\]

\[
\alpha _{2}=\frac{x^{2}-1}{2}z+\cdots \in Q(x)[[z]][\frac{1}{z}]\]
The partial fraction decomposition of  \( G(zt,\frac{z}{t}) \) reads
\[
G(zt,\frac{z}{t})=-\frac{2}{z^{3}}\cdot \frac{1}{\alpha _{1}-\alpha _{2}}\cdot \left( \frac{\alpha _{1}}{t-\alpha _{1}}-\frac{\alpha _{2}}{t-\alpha _{2}}\right) \]
It follows
that
\[
F(z^{2})=-\frac{2}{z^{3}}\cdot \frac{1}{\alpha _{1}-\alpha _{2}}\cdot \left( \frac{\alpha _{1}}{0-\alpha _{1}}-0\right) =\frac{1}{\sqrt{1-2xz^{2}+z^{4}}}\]
hence
\[
F(z)=\frac{1}{\sqrt{1-2xz+z^{2}}}\]


It follows that  \( (1-2xz+z^{2})\cdot \frac{d}{dz}F+(z-x)\cdot F=0 \). This is equivalent to the claimed recurrence.

Starting from the closed form for  \( F \), we compute a linear relation
for the partial derivatives of  \( F \). Write  \( \partial _{x}=\frac{d}{dx} \) and  \( \Delta _{z}=z\frac{d}{dz} \). One computes
\[
F=1\cdot F\]

\[
\left( 1-2xz+z^{2}\right) \cdot \partial _{x}F=z\cdot F\]

\[
\left( 1-2xz+z^{2}\right) ^{2}\cdot \partial _{x}^{2}F=3z^{2}\cdot F\]

\[
\left( 1-2xz+z^{2}\right) \cdot \Delta _{z}F=(xz-z^{2})\cdot F\]

\[
\left( 1-2xz+z^{2}\right) ^{2}\cdot \partial _{x}\Delta _{z}F=(z+xz^{2}-2z^{3})\cdot F\]

\[
\left( 1-2xz+z^{2}\right) ^{2}\cdot \Delta _{z}^{2}F=\left( xz+(x^{2}-2)z^{2}-xz^{3}+z^{4}\right) \cdot F\]
Solve
a homogeneous  \( 5\times 6 \) system of linear equations over  \( Q(x) \) to get 
\[
\left( 1-2xz+z^{2}\right) ^{2}\cdot \left( (-2x)\cdot \partial _{x}F+(1-x^{2})\cdot \partial _{x}^{2}F+\Delta _{z}F+\Delta _{z}^{2}F\right) =0\]
Divide by
the first factor to get
\[
(-2x)\cdot \partial _{x}F+(1-x^{2})\cdot \partial _{x}^{2}F+\Delta _{z}F+\Delta _{z}^{2}F=0\]
This is equivalent to the claimed equation
 \( (1-x^{2})\cdot P_{n}^{''}(x)-2x\cdot P_{n}^{'}(x)+(n^{2}+n)\cdot P_{n}(x)=0 \).

\begin{lyxsectionbibliography}

\item [1] Bruno Haible: D-finite power series in several variables. \em Diploma
thesis, University of Karlsruhe, June 1989\em . Sections 2.14, 2.15
and 2.22.

\end{lyxsectionbibliography}

\end{document}