15.9 Relations to Other Functions15.11 Riemann’s Differential Equation

§15.10 Hypergeometric Differential Equation

Contents

§15.10(i) Fundamental Solutions

15.10.1 z(1-z)\frac{{d}^{2}w}{{dz}^{2}}+\left(c-(a+b+1)z\right)\frac{dw}{dz}-abw=0.

This is the hypergeometric differential equation. It has regular singularities at z=0,1,\infty, with corresponding exponent pairs \{ 0,1-c\}, \{ 0,c-a-b\}, \{ a,b\}, respectively. When none of the exponent pairs differ by an integer, that is, when none of c, c-a-b, a-b is an integer, we have the following pairs f_{1}(z), f_{2}(z) of fundamental solutions. They are also numerically satisfactory (§2.7(iv)) in the neighborhood of the corresponding singularity.

Singularity z=\infty

(a) If c equals n=1,2,3,\dots, and a=1,2,\dots,n-1, then fundamental solutions in the neighborhood of z=0 are given by (15.10.2) with the interpretation (15.2.5) for f_{2}(z).

(b) If c equals n=1,2,3,\dots, and a\neq 1,2,\dots,n-1, then fundamental solutions in the neighborhood of z=0 are given by \mathop{F\/}\nolimits\!\left(a,b;n;z\right) and

Moreover, in (15.10.9) and (15.10.10) the symbols a and b are interchangeable.

(c) If the parameter c in the differential equation equals 2-n=0,-1,-2,\dots, then fundamental solutions in the neighborhood of z=0 are given by z^{{n-1}} times those in (a) and (b), with a and b replaced throughout by a+n-1 and b+n-1, respectively.

(d) If a+b+1-c equals n=1,2,3,\dots, or 2-n=0,-1,-2,\dots, then fundamental solutions in the neighborhood of z=1 are given by those in (a), (b), and (c) with z replaced by 1-z.

(e) Finally, if a-b+1 equals n=1,2,3,\dots, or 2-n=0,-1,-2,\dots, then fundamental solutions in the neighborhood of z=\infty are given by z^{{-a}} times those in (a), (b), and (c) with b and z replaced by a-c+1 and \ifrac{1}{z}, respectively.

§15.10(ii) Kummer’s 24 Solutions and Connection Formulas

The three pairs of fundamental solutions given by (15.10.2), (15.10.4), and (15.10.6) can be transformed into 18 other solutions by means of (15.8.1), leading to a total of 24 solutions known as Kummer’s solutions.

15.10.11 w_{1}(z)=\mathop{F\/}\nolimits\!\left({a,b\atop c};z\right)=(1-z)^{{c-a-b}}\mathop{F\/}\nolimits\!\left({c-a,c-b\atop c};z\right)=(1-z)^{{-a}}\mathop{F\/}\nolimits\!\left({a,c-b\atop c};\frac{z}{z-1}\right)=(1-z)^{{-b}}\mathop{F\/}\nolimits\!\left({c-a,b\atop c};\frac{z}{z-1}\right).
15.10.12 w_{2}(z)={z^{{1-c}}}\mathop{F\/}\nolimits\!\left({a-c+1,b-c+1\atop 2-c};z\right)={z^{{1-c}}(1-z)^{{c-a-b}}}\*\mathop{F\/}\nolimits\!\left({1-a,1-b\atop 2-c};z\right)={z^{{1-c}}(1-z)^{{c-a-1}}}\*\mathop{F\/}\nolimits\!\left({a-c+1,1-b\atop 2-c};\frac{z}{z-1}\right)={z^{{1-c}}(1-z)^{{c-b-1}}}\*\mathop{F\/}\nolimits\!\left({1-a,b-c+1\atop 2-c};\frac{z}{z-1}\right).
15.10.13 w_{3}(z)=\mathop{F\/}\nolimits\!\left({a,b\atop a+b-c+1};1-z\right)=z^{{1-c}}\mathop{F\/}\nolimits\!\left({a-c+1,b-c+1\atop a+b-c+1};1-z\right)=z^{{-a}}\mathop{F\/}\nolimits\!\left({a,a-c+1\atop a+b-c+1};1-\frac{1}{z}\right)=z^{{-b}}\mathop{F\/}\nolimits\!\left({b,b-c+1\atop a+b-c+1};1-\frac{1}{z}\right).
15.10.14 w_{4}(z)=(1-z)^{{c-a-b}}\mathop{F\/}\nolimits\!\left({c-a,c-b\atop c-a-b+1};1-z\right)=z^{{1-c}}(1-z)^{{c-a-b}}\mathop{F\/}\nolimits\!\left({1-a,1-b\atop c-a-b+1};1-z\right)=z^{{a-c}}(1-z)^{{c-a-b}}\mathop{F\/}\nolimits\!\left({1-a,c-a\atop c-a-b+1};1-\frac{1}{z}\right)=z^{{b-c}}(1-z)^{{c-a-b}}\mathop{F\/}\nolimits\!\left({1-b,c-b\atop c-a-b+1};1-\frac{1}{z}\right).
15.10.15 w_{5}(z)=e^{{a\pi i}}z^{{-a}}\*\mathop{F\/}\nolimits\!\left({a,a-c+1\atop a-b+1};\frac{1}{z}\right)=e^{{(c-b)\pi i}}z^{{b-c}}(1-z)^{{c-a-b}}\*\mathop{F\/}\nolimits\!\left({1-b,c-b\atop a-b+1};\frac{1}{z}\right)=(1-z)^{{-a}}\mathop{F\/}\nolimits\!\left({a,c-b\atop a-b+1};\frac{1}{1-z}\right)=e^{{(c-1)\pi i}}z^{{1-c}}(1-z)^{{c-a-1}}\*\mathop{F\/}\nolimits\!\left({1-b,a-c+1\atop a-b+1};\frac{1}{1-z}\right).
15.10.16 w_{6}(z)=e^{{b\pi i}}z^{{-b}}\mathop{F\/}\nolimits\!\left({b,b-c+1\atop b-a+1};\frac{1}{z}\right)=e^{{(c-a)\pi i}}z^{{a-c}}(1-z)^{{c-a-b}}\*\mathop{F\/}\nolimits\!\left({1-a,c-a\atop b-a+1};\frac{1}{z}\right)=(1-z)^{{-b}}\mathop{F\/}\nolimits\!\left({b,c-a\atop b-a+1};\frac{1}{1-z}\right)=e^{{(c-1)\pi i}}z^{{1-c}}(1-z)^{{c-b-1}}\*\mathop{F\/}\nolimits\!\left({1-a,b-c+1\atop b-a+1};\frac{1}{1-z}\right).

The \binom{6}{3}=20 connection formulas for the principal branches of Kummer’s solutions are: