15 Hypergeometric FunctionProperties

§15.5 Derivatives and Contiguous Functions

Contents
  1. §15.5(i) Differentiation Formulas
  2. §15.5(ii) Contiguous Functions

§15.5(i) Differentiation Formulas

15.5.5 (z⁢ddz⁡z)n⁢(zc−a−1⁢(1−z)a+b−c⁢F⁡(a,b;c;z))=(c−a)n⁢zc−a+n−1⁢(1−z)a−n+b−c⁢F⁡(a−n,b;c;z).
15.5.7 ((1−z)⁢ddz⁡(1−z))n⁢((1−z)a−1⁢F⁡(a,b;c;z))=(−1)n⁢(a)n⁢(c−b)n(c)n⁢(1−z)a+n−1⁢F⁡(a+n,b;c+n;z).
15.5.8 ((1−z)⁢ddz⁡(1−z))n⁢(zc−1⁢(1−z)b−c⁢F⁡(a,b;c;z))=(c−n)n⁢zc−n−1⁢(1−z)b−c+n⁢F⁡(a−n,b;c−n;z).

Other versions of several of the identities in this subsection can be constructed with the aid of the operator identity

15.5.10 (z⁢ddz⁡z)n=zn⁢dndzn⁡zn,
n=1,2,3,….

See Erdélyi et al. (1953a, pp. 102–103).

§15.5(ii) Contiguous Functions

The six functions F⁡(a±1,b;c;z), F⁡(a,b±1;c;z), F⁡(a,b;c±1;z) are said to be contiguous to F⁡(a,b;c;z).

15.5.11 (c−a)⁢F⁡(a−1,b;c;z)+(2⁢a−c+(b−a)⁢z)⁢F⁡(a,b;c;z)+a⁢(z−1)⁢F⁡(a+1,b;c;z) =0,
15.5.12 (b−a)⁢F⁡(a,b;c;z)+a⁢F⁡(a+1,b;c;z)−b⁢F⁡(a,b+1;c;z) =0,
15.5.13 (c−a−b)⁢F⁡(a,b;c;z)+a⁢(1−z)⁢F⁡(a+1,b;c;z)−(c−b)⁢F⁡(a,b−1;c;z) =0,
15.5.14 c⁢(a+(b−c)⁢z)⁢F⁡(a,b;c;z)−a⁢c⁢(1−z)⁢F⁡(a+1,b;c;z)+(c−a)⁢(c−b)⁢z⁢F⁡(a,b;c+1;z) =0,
15.5.15 (c−a−1)⁢F⁡(a,b;c;z)+a⁢F⁡(a+1,b;c;z)−(c−1)⁢F⁡(a,b;c−1;z) =0,
15.5.16 c⁢(1−z)⁢F⁡(a,b;c;z)−c⁢F⁡(a−1,b;c;z)+(c−b)⁢z⁢F⁡(a,b;c+1;z) =0,
15.5.16_5 F⁡(a,b;c;z)−F⁡(a−1,b;c;z)−(b/c)⁢z⁢F⁡(a,b+1;c+1;z) =0,
15.5.17 (a−1+(b+1−c)⁢z)⁢F⁡(a,b;c;z)+(c−a)⁢F⁡(a−1,b;c;z)−(c−1)⁢(1−z)⁢F⁡(a,b;c−1;z) =0,
15.5.18 c⁢(c−1)⁢(z−1)⁢F⁡(a,b;c−1;z)+c⁢(c−1−(2⁢c−a−b−1)⁢z)⁢F⁡(a,b;c;z)+(c−a)⁢(c−b)⁢z⁢F⁡(a,b;c+1;z) =0.

By repeated applications of (15.5.11)–(15.5.18) any function F⁡(a+k,b+ℓ;c+m;z), in which k,ℓ,m are integers, can be expressed as a linear combination of F⁡(a,b;c;z) and any one of its contiguous functions, with coefficients that are rational functions of a,b,c, and z.

An equivalent equation to the hypergeometric differential equation (15.10.1) is

15.5.19 z⁢(1−z)⁢(a+1)⁢(b+1)⁢F⁡(a+2,b+2;c+2;z)+(c−(a+b+1)⁢z)⁢(c+1)⁢F⁡(a+1,b+1;c+1;z)−c⁢(c+1)⁢F⁡(a,b;c;z)=0.

Further contiguous relations include:

15.5.20 z⁢(1−z)⁢(dF⁡(a,b;c;z)/dz)=(c−a)⁢F⁡(a−1,b;c;z)+(a−c+b⁢z)⁢F⁡(a,b;c;z)=(c−b)⁢F⁡(a,b−1;c;z)+(b−c+a⁢z)⁢F⁡(a,b;c;z),
15.5.21 c⁢(1−z)⁢(dF⁡(a,b;c;z)/dz)=(c−a)⁢(c−b)⁢F⁡(a,b;c+1;z)+c⁢(a+b−c)⁢F⁡(a,b;c;z).