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# 5.11: Legendre Polynomials

In this section we cover just enough about Legendre polynomials to be useful in the following section. Before starting, I want you to expand the following expression, by the binomial theorem, for $$| x |< 1$$, up to $$x^4$$ :

$\dfrac{1}{(1-2x \cos θ + x^2)^{1/2}}. \label{5.11.1} \tag{5.11.1}$

Please do go ahead and do it. Well, you probably won’t, so I’d better do it myself:

$(1-X)^{-1/2} = 1 + \dfrac{1}{2} X + \dfrac{3}{8} X^2 + \dfrac{5}{16} X^3 + \dfrac{35}{128} X^4 ... \label{5.11.2} \tag{5.11.2}$

and therefore

$[1-x(2\cos θ - x)]^{-1/2} = 1 + \dfrac{1}{2} x (2\cos θ - x) + \dfrac{3}{8} x^2 (2\cos θ - x)^2 + \dfrac{5}{16} x^3 (2\cos θ - x)^3 + \dfrac{35}{128} x^4 (2\cos θ - x)^4 ... \label{5.11.3} \tag{5.11.3}$

$= 1 + x \cos θ - \dfrac{1}{2} x^2 + \dfrac{3}{8}x^2 (4 \cos^2 θ - 4x \cos θ + x^2 ) + \dfrac{5}{16} x^3 (8 \cos^3 θ - 12x \cos^2 θ + 6x^2 \cos θ - x^3) + \dfrac{35}{128} x (16 \cos^4 θ - 32x \cos^3 θ + 24x^2 \cos^2 θ - 8x^3 \cos θ + x^4 ) ... \label{5.11.4} \tag{5.11.4}$

$= 1 + x \cos θ + x^2 (-\dfrac{1}{2} + \dfrac{3}{2} \cos^2 θ ) + x^3 (-\dfrac{3}{2} \cos θ + \dfrac{5}{2} \cos^3 θ ) + x^4 (\dfrac{3}{8} - \dfrac{15}{4} \cos^2 θ + \dfrac{35}{8} \cos^4 θ)... \label{5.11.5} \tag{5.11.5}$

The coefficients of the powers of $$x$$ are the Legendre polynomials $$P_l(\cos θ )$$, so that

$\dfrac{1}{(1-2x \cos θ + x^2)^{1/2}} = 1 + x P_1 ( \cos θ) + x^2 P_2 (\cos θ) + x^3 P_3 (\cos θ) + x^4 P_4 (\cos θ) + ... \label{5.11.6} \tag{5.11.6}$

The Legendre polynomials with argument $$\cos θ$$ can be written as series of terms in powers of $$\cos θ$$ by substitution of $$\cos θ$$ for $$x$$ in equations 1.12.5 in Section 1.12 of Chapter 1. Note that $$x$$ in Section 1 is not the same as $$x$$ in the present section. Alternatively they can be written as series of cosines of multiples of $$θ$$ as follows.

\begin{array}{l}
P_0 = 1 \\
P_1 = \cos θ \\
P_2 = \dfrac{1}{4} (3\cos 2θ + 1) \\
P_3 = \dfrac{1}{8} (5\cos 3θ + 3\cos θ) \\
P_4 = \dfrac{1}{64} (35 \cos 4 θ + 20 \cos 2 θ + 9 ) \\
P_5 = \dfrac{1}{128} (63 \cos 5 θ + 35 \cos 3 θ + 30 \cos θ) \\
P_6 = \dfrac{1}{512} (231 \cos 6 θ + 126 \cos 4θ + 105 \cos 2θ + 50) \\
P_7 = \dfrac{1}{1024} (429 \cos 7θ + 231 \cos 5θ + 189 \cos 3θ + 175 \cos θ) \\
P_8 = (6435 \cos 8θ + 3432 \cos 6θ + 2772 \cos 4θ + 2520 \cos 2θ + 1225)/2^{14} \\
\label{5.11.7} \tag{5.11.7}
\end{array}

For example, $$P_6(\cos θ)$$ can be written either as given by equation \ref{5.11.7}, or as given by equation 1, namely

$P_6 = \dfrac{1}{16} (231c^6 - 315 c^4 + 105c^2 - 5), \text{ where } c = \cos θ. \label{5.11.8} \tag{5.11.8}$

The former may look neater, and the latter may look “awkward” because of all the powers. However, the latter is far faster to compute, particularly when written as nested parentheses:

$P_6 = (-5 + C(105 + C(-315 + 231C)))/16, \text{ where } C = \cos^2 θ. \label{5.11.9} \tag{5.11.9}$