Kummer's Hypergeometric Theorem

Theorem
Let $x, n \in \C$.

Let $n \notin \Z_{< 0}$.

Let $\map \Re {x + 1} > 0$.

Then:
 * $\map F {n, -x; x + n + 1; -1} = \dfrac {\map \Gamma {x + n + 1} \map \Gamma {\dfrac n 2 + 1} } {\map \Gamma {x + \dfrac n 2 + 1} \map \Gamma {n + 1} }$

where:
 * $\map F {n, -x; x + n + 1; -1}$ is the Gaussian hypergeometric function of $-1$
 * $\map \Gamma {n + 1} = n!$ is the Gamma function.

Proof
First we note the definition of Gaussian hypergeometric function:


 * $\map F {n, -x; x + n + 1; -1} = \ds \sum_{k \mathop = 0}^\infty \dfrac { n^{\overline k} \paren {-x}^{\overline k} } {\paren {x + n + 1}^{\overline k} } \dfrac {\paren {-1}^k} {k!}$

where $x^{\overline k}$ denotes the $k$th rising factorial power of $x$.

Two lemmata:

Lemma 2
We use Dixon's Hypergeometric Theorem:


 * $\ds \map { {}_3 \operatorname F_2} { { {n, -x, -y} \atop {x + n + 1, y + n + 1} } \, \middle \vert \, 1} = \dfrac {\map \Gamma {x + n + 1} \map \Gamma {y + n + 1} \map \Gamma {\dfrac n 2 + 1} \map \Gamma {x + y + \dfrac n 2 + 1} } { \map \Gamma {n + 1} \map \Gamma {x + y + n + 1} \map \Gamma {x + \dfrac n 2 + 1} \map \Gamma {y + \dfrac n 2 + 1} }$

where $\ds \map { {}_3 \operatorname F_2} { { {n, -x, -y} \atop {x + n + 1, y + n + 1} } \, \middle \vert \, 1}$ is the generalized hypergeometric function of $1$.

So:

Also see

 * Dixon's Hypergeometric Theorem
 * Dougall's Hypergeometric Theorem
 * Gauss's Hypergeometric Theorem