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Associated Legendre function
In mathematics, the associated Legendre functions are the canonical solutions of the general Legendre equation
where the indices 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 and m are integers with 0 ≤ m ≤ , or with trivially equivalent negative values. When in addition m is even, the function is a polynomial. When m is zero and integer, these functions are identical to the Legendre polynomials.
This 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 equations) in spherical coordinates.
Additional recommended knowledge
These functions are denoted . 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)
The ( − 1)m factor in this formula is known as the Condon-Shortley phase. Some authors omit it.
Since, by Rodrigues' formula,
This equation allows extension of the range of m to: -l ≤ m ≤ l. The definitions of Pl(±m), 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
then it follows that the proportionality constant is
Assuming , they satisfy the orthogonality condition for fixed m:
Where is the Kronecker delta.
Also, they satisfy the orthogonality condition for fixed :
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:
(This followed from the Rodrigues' formula definition. This definition also makes the various recurrence formulas work for positive or negative m.)
The differential equation is also invariant under a change from to , and the functions for negative are defined by
The first few associated Legendre polynomials
The first few associated Legendre polynomials, including those for negative values of m, are:
These functions have a number of recurrence properties:
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 
This formula is to be used under the following assumptions:
Other quantities appearing in the formula are defined as
The integral is zero unless
The Legendre functions, and the hypergeometric function
These functions may be defined for general complex parameters and argument:
where Γ is the gamma function and is the hypergeometric function
They are called the Legendre functions when defined in this more general way. They satisfy the same differential equation as before:
Since this is a second order differential equation, it has a second solution, , defined as:
and 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 x = cosθ:
The first few polynomials, parameterized this way, are:
For fixed m, are orthogonal, parameterized by θ over [0,π], with weight sinθ:
Also, for fixed :
In terms of θ, are solutions of
More precisely, given an integer m0, the above equation has nonsingular solutions only when for an integer, and those solutions are proportional to .
Applications in physics: 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 θ used above. The longitude angle, φ, appears in a multiplying factor. Together, they make a set of functions called spherical harmonics.
These functions express the symmetry of the 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 spaces.
What makes these functions useful is that they are central to the solution of the equation on the surface of a sphere. In spherical coordinates θ (colatitude) and φ (longitude), the Laplacian is
When the partial differential equation
is solved by the method of separation of variables, one gets a φ-dependent part sin(mφ) or cos(mφ) for integer m≥0, and an equation for the θ-dependent part
for which the solutions are with and .
Therefore, the equation
has nonsingular separated solutions only when , and those solutions are proportional to
For each choice of , there are functions for the various values of m and choices of sine and cosine. They are all orthogonal in both and m when integrated over the surface of the sphere.
The solutions are usually written in terms of complex exponentials:
The functions 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
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 , and hence the solutions are spherical harmonics.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Associated_Legendre_function". A list of authors is available in Wikipedia.|