How to prove that $p! \ \ (mod \ p^2) \ = p(p-1) $

Question

While doing my Math homework about Modular arithmetic. I accidentally found this

$p!\equiv p(p-1)\pmod{p^2}$

It's help me save time to find $ \ 21! \ \ (mod \ 361) \ $ a lot.

The question is how can I prove an equation above . Thank you in advance.

Answer 1

Warning ! You statement is true only when $n$ is prime. For example for $n=4$, you don't have $n! = n(n-1) \quad (\mathrm{mod} \text{ } n^2)$. So I guess you can't use it with $21$ ("by hand", you can see that $21!=323 \neq 21\times 20 \quad (\mathrm{mod} \text{ } 361)$).

Proof when $n$ is prime :

By Wilson's theorem, $$(p-1)! = -1 \quad (\mathrm{mod} \text{ } p)$$

Multiplying by $p$, this implies $$p! = -p \quad (\mathrm{mod} \text{ } p^2)$$

i.e. $$p! = p^2-p = p(p-1) \quad (\mathrm{mod} \text{ } p^2)$$

Answer 2

Wilson's Theorem states that:

$(p-1)! = -1 (mod p^{2})$ ......(a)

and

$p!=p(p-1)!$, $-1=p-1(mod p^{2})$ .......(b)

applying (b) to (a), we have

$p!=p(p-1)!=p*-1 (mod p^{2})$

which implies that

$p!=p(p-1) (mod p^{2})$

Note that the $p$ above stands for prime numbers and $21$ is not a prime number since $21=3*7$. Therefore the above statements will not apply, but take for example, 5 is a prime number and

$5!=5*4*3*2*1=120=5(5-1)=20 (mod 25)$

Answer 3

$$p! \equiv p(p-1) \pmod{p^2} \Leftrightarrow p^2 \mid p!-p(p-1) \Leftrightarrow p \mid (p-1)!-(p-1) \Leftrightarrow (p-1)! \equiv -1 \pmod{p}$$ If $p$ is a prime, then except for the two solutions of $x^2 \equiv 1 \pmod{p}$ (which are equal $\pmod{p}$ for $p=2$), we can pair the numbers in $2, ... (p-2)$ in pairs $(x,y)$ with $x \not\equiv y \pmod{p}$ such that $xy \equiv 1 \pmod{p}$, so $(p-2)! \equiv 1 \pmod{p}$ and so $(p-1)! \equiv (p-1)(p-2)! \equiv (p-1) \equiv -1 \pmod{p}$.