Associated Legendre function: Difference between revisions

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\left[ \ell(\ell+1) - \frac{m^2}{1-x^2}\right] P^{m}_\ell(x)= 0 .
\left[ \ell(\ell+1) - \frac{m^2}{1-x^2}\right] P^{m}_\ell(x)= 0 .
</math>
</math>
One often finds the equation written in the following equivalent way
:<math>
\left( (1-x^{2})\; y\,' \right)' +\left( \ell(\ell+1)
-\frac{m^{2} }{1-x^{2} } \right) y=0, 
</math>
where the primes indicate differentiation with respect to ''x''.
In physical applications it is usually the case that  ''x'' = cos&theta;, then the  associated Legendre differential equation takes the form
In physical applications it is usually the case that  ''x'' = cos&theta;, then the  associated Legendre differential equation takes the form
:<math>
:<math>

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In mathematics and physics, an associated Legendre function Plm is related to a Legendre polynomial Pl by the following equation

For even m the associated Legendre function is a polynomial, for odd m the function contains the factor (1−x ² )½ and hence is not a polynomial.

The associated Legendre functions are important in quantum mechanics and potential theory.

According to Ferrers[1] the polynomials were named in 1875 "Associated Legendre functions" by the British mathematician Isaac Todhunter,[2] where "associated function" is Todhunter's translation of the German term zugeordnete Function, coined in 1861 by Heine[3] and "Legendre" is in honor of the French mathematician Adrien-Marie Legendre (1752–1833), who was the first to introduce and study the polynomials Pl(x).

Differential equation

Define

where Pl(x) is a Legendre polynomial. Differentiating the Legendre differential equation:

m times gives an equation for Πml

After substitution of

and after multiplying through with , we find the associated Legendre differential equation:

One often finds the equation written in the following equivalent way

where the primes indicate differentiation with respect to x.

In physical applications it is usually the case that x = cosθ, then the associated Legendre differential equation takes the form

Extension to negative m

By the Rodrigues formula, one obtains

This equation allows extension of the range of m to: .

Since the associated Legendre equation is invariant under the substitution m → −m, the equations for Pl ±m, resulting from this expression, are proportional.

To obtain the proportionality constant we consider

and we bring the factor (1−x²)m/2 to the other side. Equate the coefficient of the highest power of x on the left and right hand side of

and it follows that the proportionality constant is

so that the associated Legendre functions of same |m| are related to each other by

Note that the phase factor (−1)m arising in this expression is not due to some arbitrary phase convention, but arises from expansion of (1−x²)m.

Orthogonality relations

Important integral relations are:

(see the subpage Proofs for a detailed proof of this relation) and:

Recurrence relations

The functions satisfy the following difference equations, which are taken from Edmonds.[4]

Reference

  1. N. M. Ferrers, An Elementary Treatise on Spherical Harmonics, MacMillan, 1877 (London), p. 77. Online.
  2. I. Todhunter, An Elementary Treatise on Laplace's, Lamé's, and Bessel's Functions, MacMillan, 1875 (London). In fact, Todhunter called the Legendre polynomials "Legendre coefficients".
  3. E. Heine, Handbuch der Kugelfunctionen, G. Reimer, 1861 (Berlin).Google book online
  4. A. R. Edmonds, Angular Momentum in Quantum Mechanics, Princeton University Press, 2nd edition (1960)