In
mathematicsMathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...
, the
associated Legendre polynomials are the canonical solutions of the
general Legendre equation
or equivalently
where the indices ℓ and m
(which are integers) are referred to as the degree and order of the associated Legendre polynomial respectively. This equation has nonzero solutions that are nonsingular on [−1, 1] only if ℓ and m
are integers with 0 ≤ m
≤ ℓ, or with trivially equivalent negative values. When in addition m
is even, the function is a polynomialIn mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and nonnegative integer exponents...
. When m
is zero and ℓ integer, these functions are identical to the Legendre polynomials. In general, when ℓ and m
are integers, the regular solutions are sometimes called "associated Legendre polynomials", even though they are not polynomialIn mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and nonnegative integer exponents...
s when m
is odd. The fully general class of functions with arbitrary real or complex values of ℓ and m are
Legendre functionIn mathematics, the Legendre functions Pλ, Qλ and associated Legendre functions P, Q are generalizations of Legendre polynomials to noninteger degree.Differential equation:...
s. In that case the parameters are usually labelled with Greek letters.
The Legendre
ordinary differential equationIn mathematics, an ordinary differential equation is a relation that contains functions of only one independent variable, and one or more of their derivatives with respect to that variable....
is frequently encountered in
physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and other technical fields. In particular, it occurs when solving
Laplace's equationIn mathematics, Laplace's equation is a secondorder partial differential equation named after PierreSimon Laplace who first studied its properties. This is often written as:where ∆ = ∇² is the Laplace operator and \varphi is a scalar function...
(and related
partial differential equationIn mathematics, partial differential equations are a type of differential equation, i.e., a relation involving an unknown function of several independent variables and their partial derivatives with respect to those variables...
s) in spherical coordinates. Associated Legendre polynomials play a vital role in the definition of
spherical harmonicsIn mathematics, spherical harmonics are the angular portion of a set of solutions to Laplace's equation. Represented in a system of spherical coordinates, Laplace's spherical harmonics Y_\ell^m are a specific set of spherical harmonics that forms an orthogonal system, first introduced by Pierre...
.
Definition for nonnegative integer parameters ℓ and m
These functions are denoted , where the superscript indicates the order, and not a power of P
. Their most straightforward definition is in terms
of derivatives of ordinary Legendre polynomials (m ≥ 0)
The
factor in this formula is known as the Condon–Shortley phase. Some authors omit it. That the functions described by this equation satisfy the general Legendre differential equation with the indicated values of the parameters ℓ and m
follows by differentiating m
times the Legendre equation for P
_{ℓ}:
Moreover, since by Rodrigues' formulaIn mathematics, Rodrigues's formula is a formula for Legendre polynomials independently introduced by , and...
,
the P
can be expressed in the form
This equation allows extension of the range of m
to: −ℓ ≤ m
≤ ℓ. The definitions of P
_{ℓ}^{±}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
so that
Alternative notations
The following alternative notations are also used in literature:
Orthogonality
Assuming
, they satisfy the orthogonality condition for fixed m:
Where
is the
Kronecker delta.
Also, they satisfy the orthogonality condition for fixed ℓ:
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 ℓ to
, and the functions for negative ℓ are defined by
.
The first few associated Legendre functions
The first few associated Legendre functions, including those for negative values of m, are:
Recurrence formula
These functions have a number of recurrence properties:
Helpful identities (initial values for the first recursion):
with !! the double factorial.
Gaunt's formula
The integral over the product of three associated Legendre polynomials (with orders matching as shown below) is a necessary ingredient when developing products of Legendre polynomials into a series linear in the Legendre polynomials. For instance, this 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:
 the degrees are nonnegative integers
 all three orders are nonnegative integers
 is the largest of the three orders
 the orders sum up
 the degrees obey
Other quantities appearing in the formula are defined as



The integral is zero unless
 the sum of degrees is even so that is an integer
 the triangular condition is satisfied
Generalization via hypergeometric functions
These functions may actually be defined for general complex parameters and argument:
where
is the
gamma functionIn mathematics, the gamma function is an extension of the factorial function, with its argument shifted down by 1, to real and complex numbers...
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
:
The first few polynomials, parameterized this way, are:
For fixed
m,
are orthogonal, parameterized by θ over
, with weight
:
Also, for fixed ℓ:
In terms of θ,
are solutions of
More precisely, given an integer
m0, the above equation has
nonsingular solutions only when
for ℓ
an integer ≥
m, and those solutions are proportional to
.
Applications in physics: spherical harmonics
In many occasions in
physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, associated Legendre polynomials in terms of angles occur where spherical
symmetrySymmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...
is involved. The colatitude angle in spherical coordinates is
the angle
used above. The longitude angle,
, appears in a multiplying factor. Together, they make a set of functions called
spherical harmonicSpherical Harmonic is a science fiction novel from the Saga of the Skolian Empire by Catherine Asaro. It tells the story of Dyhianna Selei , the Ruby Pharaoh of the Skolian Imperialate, as she strives to reform her government and reunite her family in the aftermath of a devastating interstellar...
s. These functions express the symmetry of the
twosphereIn mathematics, the Riemann sphere , named after the 19th century mathematician Bernhard Riemann, is the sphere obtained from the complex plane by adding a point at infinity...
under the action of the
Lie groupIn mathematics, a Lie group is a group which is also a differentiable manifold, with the property that the group operations are compatible with the smooth structure...
SO(3).
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 equationIn mathematics, partial differential equations are a type of differential equation, i.e., a relation involving an unknown function of several independent variables and their partial derivatives with respect to those variables...
is solved by the method of separation of variables, one gets a φdependent part
or
for integer m≥0, and an equation for the θdependent part
for which the solutions are
with
and
.
Therefore, the equation
has nonsingular separated solutions only when
,
and those solutions are proportional to
and
For each choice of ℓ, there are functions
for the various values of
m and choices of sine and cosine.
They are all orthogonal in both ℓ 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 harmonicsIn mathematics, spherical harmonics are the angular portion of a set of solutions to Laplace's equation. Represented in a system of spherical coordinates, Laplace's spherical harmonics Y_\ell^m are a specific set of spherical harmonics that forms an orthogonal system, first introduced by Pierre...
, 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 seriesIn mathematics, a Fourier series decomposes periodic functions or periodic signals into the sum of a set of simple oscillating functions, namely sines and cosines...
. 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 harmonicsIn mathematics, spherical harmonics are the angular portion of a set of solutions to Laplace's equation. Represented in a system of spherical coordinates, Laplace's spherical harmonics Y_\ell^m are a specific set of spherical harmonics that forms an orthogonal system, first introduced by Pierre...
).
When a 3dimensional 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.
Generalizations
The Legendre polynomials are closely related to
hypergeometric seriesIn mathematics, a generalized hypergeometric series is a series in which the ratio of successive coefficients indexed by n is a rational function of n. The series, if convergent, defines a generalized hypergeometric function, which may then be defined over a wider domain of the argument by...
. In the form of spherical harmonics, they express the symmetry of the
twosphereIn mathematics, the Riemann sphere , named after the 19th century mathematician Bernhard Riemann, is the sphere obtained from the complex plane by adding a point at infinity...
under the action of the
Lie groupIn mathematics, a Lie group is a group which is also a differentiable manifold, with the property that the group operations are compatible with the smooth structure...
SO(3). There are many other Lie groups besides SO(3), and an analogous generalization of the Legendre polynomials exist to express the symmetries of semisimple Lie groups and
Riemannian symmetric spaceIn differential geometry, representation theory and harmonic analysis, a symmetric space is a smooth manifold whose group of symmetries contains an inversion symmetry about every point. There are two ways to formulate the inversion symmetry, via Riemannian geometry or via Lie theory...
s. Crudely speaking, one may define a Laplacian on symmetric spaces; the eigenfunctions of the Laplacian can be thought of as generalizations of the spherical harmonics to other settings.
See also
 Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
 Gaussian quadrature
In numerical analysis, a quadrature rule is an approximation of the definite integral of a function, usually stated as a weighted sum of function values at specified points within the domain of integration....
 Legendre polynomials
 Spherical harmonic
Spherical Harmonic is a science fiction novel from the Saga of the Skolian Empire by Catherine Asaro. It tells the story of Dyhianna Selei , the Ruby Pharaoh of the Skolian Imperialate, as she strives to reform her government and reunite her family in the aftermath of a devastating interstellar...
s
 Whipple's transformation of Legendre functions
External links