# $(p\!-\!1\!-\!h)!\,h! \equiv (-1)^{h+1}\!\!\pmod{\! p}\,$ [Wilson Reflection Formula]

Suppose that $p$ is a prime. Suppose further that $h$ and $k$ are non-negative integers such that $h + k = p − 1$.

I want to prove that $h!k! + (−1)^h \equiv 0 \pmod{p}$

My first thought is that by Wilson's theorem, $(p-1)! \equiv -1 \pmod{p}$, and $h!k!$ divides $(p-1)!$ (definition of a binomial). Where would I go from here?

Wilson's theorem $$\Rightarrow$$ any complete system of representatives $$\,r_i\,$$ of $$\rm\color{#c00}{nonzero}$$ remainders mod $$\,p\,$$ has product $$\equiv -1,\,$$ by $$\,r_i\equiv i\,\Rightarrow\,\displaystyle \prod_{i=1}^{p-1} r_i\equiv \prod_{i=1}^{p-1} i \equiv (p-1)!\equiv -1\,$$ by inductive extension of Congruence Product Rule. In particular this is true for any sequence of $$\,p\,$$ consecutive integers, after removing its unique $$\rm\color{#c00}{multiple}$$ of $$\,p.\,$$ Your special case is the sequence $$\, \underbrace{\color{#90f}{-h},\,-h\!+\!1,\ldots,\color{#0a0}{-1}}_{\!\!\textstyle\equiv\,\color{#90f}{k\!+\!1},\,k\!+\!2,\cdots,\color{#0a0}{p\!-\!1}}\!\!\!\!,\require{cancel}\color{#c00}{\cancel{0,}} 1,2,\ldots, k\ \ \ \text{whose product is}\,\ \ (-1)^h h!\,k!\equiv -1\qquad$$

since $$\,\color{#90f}{-h\equiv k\!+\!1}\,$$ by $$\,h\!+\!k\!+\!1\equiv p\equiv 0$$

Remark $$\$$ This is slight reformulation of the Wilson reflection formula mentioned yesterday

$$k! = (p\!-\!1\!-\!h)! \equiv \frac{(-1)^{h+1}}{h!}\!\!\pmod{\! p},\,\ \ 0\le h< p\ {\rm prime}\qquad$$

Use the fact that

$$h! = (-1)^h (p-1)(p-2) \dots (p-h) \mod p$$

• Ah, so h!k! becomes [(-1)^h](p-1)! mod p, which is just -(-1)^h. Thanks so much! Feb 27 '11 at 22:59