Plot of the Jacobi polynomial function with and and in the complex plane from to with colors created with Mathematica 13.1 function ComplexPlot3D
In mathematics, Jacobi polynomials (occasionally called hypergeometric polynomials) are a class of classical orthogonal polynomials. They are orthogonal with respect to the weight on the interval . The Gegenbauer polynomials, and thus also the Legendre, Zernike and Chebyshev polynomials, are special cases of the Jacobi polynomials.[1]
The Jacobi polynomials are defined via the hypergeometric function as follows:[2]
where is Pochhammer's symbol (for the falling factorial). In this case, the series for the hypergeometric function is finite, therefore one obtains the following equivalent expression:
Rodrigues' formula
An equivalent definition is given by Rodrigues' formula:[1][3]
If , then it reduces to the Legendre polynomials:
Alternate expression for real argument
For real the Jacobi polynomial can alternatively be written as
and for integer
where is the gamma function.
In the special case that the four quantities , , , are nonnegative integers, the Jacobi polynomial can be written as
(1)
The sum extends over all integer values of for which the arguments of the factorials are nonnegative.
Special cases
Basic properties
Orthogonality
The Jacobi polynomials satisfy the orthogonality condition
As defined, they do not have unit norm with respect to the weight. This can be corrected by dividing by the square root of the right hand side of the equation above, when .
Although it does not yield an orthonormal basis, an alternative normalization is sometimes preferred due to its simplicity:
Symmetry relation
The polynomials have the symmetry relation
thus the other terminal value is
Derivatives
The th derivative of the explicit expression leads to
for . Writing for brevity , and , this becomes in terms of
Since the Jacobi polynomials can be described in terms of the hypergeometric function, recurrences of the hypergeometric function give equivalent recurrences of the Jacobi polynomials. In particular, Gauss' contiguous relations correspond to the identities
Generating function
The generating function of the Jacobi polynomials is given by
where
and the branch of square root is chosen so that .[1]
Asymptotics of Jacobi polynomials
For in the interior of , the asymptotics of for large is given by the Darboux formula[1]
where
and the "" term is uniform on the interval for every .
The asymptotics of the Jacobi polynomials near the points is given by the Mehler–Heine formula
where the limits are uniform for in a bounded domain.
The asymptotics outside is less explicit.
Applications
Wigner d-matrix
The expression (1) allows the expression of the Wigner d-matrix (for ) in terms of Jacobi polynomials:[4]
^ abcdefSzegő, Gábor (1939). "IV. Jacobi polynomials.". Orthogonal Polynomials. Colloquium Publications. Vol. XXIII. American Mathematical Society. ISBN 978-0-8218-1023-1. MR 0372517. The definition is in IV.1; the differential equation – in IV.2; Rodrigues' formula is in IV.3; the generating function is in IV.4; the recurrent relation is in IV.5; the asymptotic behavior is in VIII.2
^Abramowitz, Milton; Stegun, Irene Ann, eds. (1983) [June 1964]. "Chapter 22". Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. Applied Mathematics Series. Vol. 55 (Ninth reprint with additional corrections of tenth original printing with corrections (December 1972); first ed.). Washington D.C.; New York: United States Department of Commerce, National Bureau of Standards; Dover Publications. p. 561. ISBN 978-0-486-61272-0. LCCN 64-60036. MR 0167642. LCCN 65-12253.
Andrews, George E.; Askey, Richard; Roy, Ranjan (1999), Special functions, Encyclopedia of Mathematics and its Applications, vol. 71, Cambridge University Press, ISBN 978-0-521-62321-6, MR 1688958, ISBN 978-0-521-78988-2
Koornwinder, Tom H.; Wong, Roderick S. C.; Koekoek, Roelof; Swarttouw, René F. (2010), "Orthogonal Polynomials", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W. (eds.), NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0-521-19225-5, MR 2723248.