Gegenbauer polynomials

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In mathematics, Gegenbauer polynomials or ultraspherical polynomials C(α)
n
(x) are orthogonal polynomials on the interval [1,1] with respect to the weight function (1  x2)α–1/2. They generalize Legendre polynomials and Chebyshev polynomials, and are special cases of Jacobi polynomials. They are named after Leopold Gegenbauer.

Characterizations

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Perspective

A variety of characterizations of the Gegenbauer polynomials are available.

  • Gegenbauer polynomials are particular solutions of the Gegenbauer differential equation (Suetin 2001):
When α = 1/2, the equation reduces to the Legendre equation, and the Gegenbauer polynomials reduce to the Legendre polynomials.
When α = 1, the equation reduces to the Chebyshev differential equation, and the Gegenbauer polynomials reduce to the Chebyshev polynomials of the second kind.[1]
(Abramowitz & Stegun p. 561). Here (2α)n is the rising factorial. Explicitly,
From this it is also easy to obtain the value at unit argument:
in which represents the rising factorial of .
One therefore also has the Rodrigues formula
  • An alternative normalization sets . Assuming this alternative normalization, the derivatives of Gegenbauer are expressed in terms of Gegenbauer:[2]

Orthogonality and normalization

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Perspective

For a fixed α > -1/2, the polynomials are orthogonal on [1, 1] with respect to the weighting function (Abramowitz & Stegun p. 774)

To wit, for n  m,

They are normalized by

Applications

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Perspective

The Gegenbauer polynomials appear naturally as extensions of Legendre polynomials in the context of potential theory and harmonic analysis. The Newtonian potential in Rn has the expansion, valid with α = (n  2)/2,

When n = 3, this gives the Legendre polynomial expansion of the gravitational potential. Similar expressions are available for the expansion of the Poisson kernel in a ball (Stein & Weiss 1971).

It follows that the quantities are spherical harmonics, when regarded as a function of x only. They are, in fact, exactly the zonal spherical harmonics, up to a normalizing constant.

Gegenbauer polynomials also appear in the theory of positive-definite functions.

The Askey–Gasper inequality reads

In spectral methods for solving differential equations, if a function is expanded in the basis of Chebyshev polynomials and its derivative is represented in a Gegenbauer/ultraspherical basis, then the derivative operator becomes a diagonal matrix, leading to fast banded matrix methods for large problems.[3]

Other properties

Dirichlet–Mehler-type integral representation:[4]Laplace-type

See also

References

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