Appell series

In mathematics, Appell series are a set of four hypergeometric series F₁, F₂, F₃, F₄ of two variables that were introduced by Paul Appell (1880) and that generalize Gauss's hypergeometric series ₂F₁ of one variable. Appell established the set of partial differential equations of which these functions are solutions, and found various reduction formulas and expressions of these series in terms of hypergeometric series of one variable.

Definitions

The Appell series F₁ is defined for |x| < 1, |y| < 1 by the double series:

$F_1(a,b_1,b_2,c;x,y) = \sum_{m,n=0}^\infty \frac{(a)_{m+n} (b_1)_m (b_2)_n} {(c)_{m+n} \,m! \,n!} \,x^m y^n ~,$

where the Pochhammer symbol (q)_n represents the rising factorial:

$(q)_n = q\,(q+1) \cdots (q+n-1) = \frac{\Gamma(q+n)}{\Gamma(q)}~,$

where the second equality is true for all complex $q$ except $q=0,-1,-2,\ldots$.

For other values of x and y the function F₁ can be defined by analytic continuation.

Similarly, the function F₂ is defined for |x| + |y| < 1 by the series:

$F_2(a,b_1,b_2,c_1,c_2;x,y) = \sum_{m,n=0}^\infty \frac{(a)_{m+n} (b_1)_m (b_2)_n} {(c_1)_m (c_2)_n \,m! \,n!} \,x^m y^n ~,$

the function F₃ for |x| < 1, |y| < 1 by the series:

$F_3(a_1,a_2,b_1,b_2,c;x,y) = \sum_{m,n=0}^\infty \frac{(a_1)_m (a_2)_n (b_1)_m (b_2)_n} {(c)_{m+n} \,m! \,n!} \,x^m y^n ~,$

and the function F₄ for |x|^½ + |y|^½ < 1 by the series:

$F_4(a,b,c_1,c_2;x,y) = \sum_{m,n=0}^\infty \frac{(a)_{m+n} (b)_{m+n}} {(c_1)_m (c_2)_n \,m! \,n!} \,x^m y^n ~.$

Recurrence relations

Like the Gauss hypergeometric series ₂F₁, the Appell double series entail recurrence relations among contiguous functions. For example, a basic set of such relations for Appell's F₁ is given by:

$(a-b_1-b_2) F_1(a,b_1,b_2,c; x,y) - a \,F_1(a+1,b_1,b_2,c; x,y) + b_1 F_1(a,b_1+1,b_2,c; x,y) + b_2 F_1(a,b_1,b_2+1,c; x,y) = 0 ~,$

$c \,F_1(a,b_1,b_2,c; x,y) - (c-a) F_1(a,b_1,b_2,c+1; x,y) - a \,F_1(a+1,b_1,b_2,c+1; x,y) = 0 ~,$

$c \,F_1(a,b_1,b_2,c; x,y) + c(x-1) F_1(a,b_1+1,b_2,c; x,y) - (c-a)x \,F_1(a,b_1+1,b_2,c+1; x,y) = 0 ~,$

$c \,F_1(a,b_1,b_2,c; x,y) + c(y-1) F_1(a,b_1,b_2+1,c; x,y) - (c-a)y \,F_1(a,b_1,b_2+1,c+1; x,y) = 0 ~.$

Any other relation^[1] valid for F₁ can be derived from these four.

Similarly, all recurrence relations for Appell's F₃ follow from this set of five:

$c \,F_3(a_1,a_2,b_1,b_2,c; x,y) + (a_1+a_2-c) F_3(a_1,a_2,b_1,b_2,c+1; x,y) - a_1 F_3(a_1+1,a_2,b_1,b_2,c+1; x,y) - a_2 F_3(a_1,a_2+1,b_1,b_2,c+1; x,y) = 0 ~,$

$c \,F_3(a_1,a_2,b_1,b_2,c; x,y) - c \,F_3(a_1+1,a_2,b_1,b_2,c; x,y) + b_1 x \,F_3(a_1+1,a_2,b_1+1,b_2,c+1; x,y) = 0 ~,$

$c \,F_3(a_1,a_2,b_1,b_2,c; x,y) - c \,F_3(a_1,a_2+1,b_1,b_2,c; x,y) + b_2 y \,F_3(a_1,a_2+1,b_1,b_2+1,c+1; x,y) = 0 ~,$

$c \,F_3(a_1,a_2,b_1,b_2,c; x,y) - c \,F_3(a_1,a_2,b_1+1,b_2,c; x,y) + a_1 x \,F_3(a_1+1,a_2,b_1+1,b_2,c+1; x,y) = 0 ~,$

$c \,F_3(a_1,a_2,b_1,b_2,c; x,y) - c \,F_3(a_1,a_2,b_1,b_2+1,c; x,y) + a_2 y \,F_3(a_1,a_2+1,b_1,b_2+1,c+1; x,y) = 0 ~.$

Derivatives and differential equations

For Appell's F₁, the following derivatives result from the definition by a double series:

$\frac {\partial} {\partial x} F_1(a,b_1,b_2,c; x,y) = \frac {a b_1} {c} F_1(a+1,b_1+1,b_2,c+1; x,y) ~,$

$\frac {\partial} {\partial y} F_1(a,b_1,b_2,c; x,y) = \frac {a b_2} {c} F_1(a+1,b_1,b_2+1,c+1; x,y) ~.$

From its definition, Appell's F₁ is further found to satisfy the following system of second-order differential equations:

$\left( x(1-x) \frac {\partial^2} {\partial x^2} + y(1-x) \frac {\partial^2} {\partial x \partial y} + [c - (a+b_1+1) x] \frac {\partial} {\partial x} - b_1 y \frac {\partial} {\partial y} - a b_1 \right) F_1(x,y) = 0 ~,$

$\left( y(1-y) \frac {\partial^2} {\partial y^2} + x(1-y) \frac {\partial^2} {\partial x \partial y} + [c - (a+b_2+1) y] \frac {\partial} {\partial y} - b_2 x \frac {\partial} {\partial x} - a b_2 \right) F_1(x,y) = 0 ~.$

Similarly, for F₃ the following derivatives result from the definition:

$\frac {\partial} {\partial x} F_3(a_1,a_2,b_1,b_2,c; x,y) = \frac {a_1 b_1} {c} F_3(a_1+1,a_2,b_1+1,b_2,c+1; x,y) ~,$

$\frac {\partial} {\partial y} F_3(a_1,a_2,b_1,b_2,c; x,y) = \frac {a_2 b_2} {c} F_3(a_1,a_2+1,b_1,b_2+1,c+1; x,y) ~.$

And for F₃ the following system of differential equations is obtained:

$\left( x(1-x) \frac {\partial^2} {\partial x^2} + y \frac {\partial^2} {\partial x \partial y} + [c - (a_1+b_1+1) x] \frac {\partial} {\partial x} - a_1 b_1 \right) F_3(x,y) = 0 ~,$

$\left( y(1-y) \frac {\partial^2} {\partial y^2} + x \frac {\partial^2} {\partial x \partial y} + [c - (a_2+b_2+1) y] \frac {\partial} {\partial y} - a_2 b_2 \right) F_3(x,y) = 0 ~.$

Integral representations

The four functions defined by Appell's double series can be represented in terms of double integrals involving elementary functions only (Gradshteyn & Ryzhik 2015, §9.184). However, Émile Picard (1881) discovered that Appell's F₁ can also be written as a one-dimensional Euler-type integral:

$F_1(a,b_1,b_2,c; x,y) = \frac{\Gamma(c)} {\Gamma(a) \Gamma(c-a)} \int_0^1 t^{a-1} (1-t)^{c-a-1} (1-xt)^{-b_1} (1-yt)^{-b_2} \,\mathrm{d}t, \quad \real \,c > \real \,a > 0 ~.$

This representation can be verified by means of Taylor expansion of the integrand, followed by termwise integration.

Special cases

Picard's integral representation implies that the incomplete elliptic integrals F and E as well as the complete elliptic integral Π are special cases of Appell's F₁:

$F(\phi,k) = \int_0^\phi \frac{\mathrm{d} \theta} {\sqrt{1 - k^2 \sin^2 \theta}} = \sin (\phi) \,F_1(\tfrac 1 2, \tfrac 1 2, \tfrac 1 2, \tfrac 3 2; \sin^2 \phi, k^2 \sin^2 \phi), \quad |\real \,\phi| < \frac \pi 2 ~,$

$E(\phi, k) = \int_0^\phi \sqrt{1 - k^2 \sin^2 \theta} \,\mathrm{d} \theta = \sin (\phi) \,F_1(\tfrac 1 2, \tfrac 1 2, -\tfrac 1 2, \tfrac 3 2; \sin^2 \phi, k^2 \sin^2 \phi), \quad |\real \,\phi| < \frac \pi 2 ~,$

$\Pi(n,k) = \int_0^{\pi/2} \frac{\mathrm{d} \theta} {(1 - n \sin^2 \theta) \sqrt{1 - k^2 \sin^2 \theta}} = \frac {\pi} {2} \,F_1(\tfrac 1 2, 1, \tfrac 1 2, 1; n,k^2) ~.$

Related series

There are seven related series of two variables, Φ₁, Φ₂, Φ₃, Ψ₁, Ψ₂, Ξ₁, and Ξ₂, which generalize Kummer's confluent hypergeometric function ₁F₁ of one variable and the confluent hypergeometric limit function ₀F₁ of one variable in a similar manner. The first of these was introduced by Pierre Humbert in 1920.

Giuseppe Lauricella (1893) defined four functions similar to the Appell series, but depending on many variables rather than just the two variables x and y. These series were also studied by Appell. They satisfy certain partial differential equations, and can also be given in terms of Euler-type integrals and contour integrals.

References

1. For example, $(y-x) F_1(a, b_1+1, b_2+1,c,x,y) = y \, F_1(a,b_1,b_2+1,c,x,y) - x \, F_1(a,b_1+1,b_2,c,x,y)$

• Appell, Paul (1880). "Sur les séries hypergéométriques de deux variables et sur des équations différentielles linéaires aux dérivées partielles". Comptes rendus hebdomadaires des séances de l'Académie des sciences (in French) 90: 296–298 and 731–735. JFM 12.0296.01. (see also "Sur la série F₃(α,α',β,β',γ; x,y)" in C. R. Acad. Sci. 90, pp. 977–980)
• Appell, Paul (1882). "Sur les fonctions hypergéométriques de deux variables". Journal de Mathématiques Pures et Appliquées. (3ème série) (in French) 8: 173–216.
• Appell, Paul; Kampé de Fériet, Joseph (1926). Fonctions hypergéométriques et hypersphériques; Polynômes d'Hermite (in French). Paris: Gauthier–Villars. JFM 52.0361.13. (see p. 14)
• Askey, R. A.; Daalhuis, Adri B. Olde (2010), "Appell series", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W., NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0521192255, MR 2723248
• Bateman, H.; Erdélyi, A. (1953). Higher Transcendental Functions, Vol. I (PDF). New York: McGraw–Hill. (see p. 224)
• Gradshteyn, Izrail Solomonovich; Ryzhik, Iosif Moiseevich; Geronimus, Yuri Veniaminovich; Tseytlin, Michail Yulyevich; Jeffrey, Alan (2015) [October 2014]. "9.18.". In Zwillinger, Daniel; Moll, Victor Hugo. Table of Integrals, Series, and Products. Translated by Scripta Technica, Inc. (8 ed.). Academic Press, Inc. ISBN 0-12-384933-0. LCCN 2014010276. ISBN 978-0-12-384933-5.
• Humbert, Pierre (1920). "Sur les fonctions hypercylindriques". Comptes rendus hebdomadaires des séances de l'Académie des sciences (in French) 171: 490–492. JFM 47.0348.01.
• Lauricella, Giuseppe (1893). "Sulle funzioni ipergeometriche a più variabili". Rendiconti del Circolo Matematico di Palermo (in Italian) 7: 111–158. doi:10.1007/BF03012437. JFM 25.0756.01.
• Picard, Émile (1881). "Sur une extension aux fonctions de deux variables du problème de Riemann relativ aux fonctions hypergéométriques". Annales scientifiques de l'École Normale Supérieure. (2ème série) (in French) 10: 305–322. JFM 13.0389.01. (see also C. R. Acad. Sci. 90 (1880), pp. 1119–1121 and 1267–1269)
• Slater, Lucy Joan (1966). Generalized hypergeometric functions. Cambridge, UK: Cambridge University Press. ISBN 0-521-06483-X. MR 0201688. (there is a 2008 paperback with ISBN 978-0-521-09061-2)

External links

• Aarts, Ronald M., "Lauricella Functions", MathWorld.
• Weisstein, Eric W., "Appell Hypergeometric Function", MathWorld.