Divisibility of n-1 Factorial by Composite n

Theorem
Let $n \in \Z$ be composite.

Then:
 * $n \divides \paren {n - 1}! \iff n \ne 4$

where:
 * $\divides$ denotes divisibility
 * $n!$ denotes the factorial of $n$.

Necessary Condition
We have that $3! = 6$ and that $4$ does not divide $6$.

So in order for $n$ to divide $n - 1$ it is necessary that $n \ne 4$.

Sufficient Condition
Let $n \ne 4$ be composite.

Let $n = r s$ where:
 * $r, s \in \Z_{> 1}$
 * $r \ne s$
 * $r, s < n$

This is always possible unless $n = p^2$ for some prime number $p$.

, let $r < s$.

Then by definition of factorial:
 * $\paren {n - 1}! = 1 \times 2 \times \ldots \times r \times \ldots \times s \times \ldots \times \paren {n - 2} \times \paren {n - 1}$

and so:
 * $n = r s \divides \paren {n - 1}!$

Let $n = p^2$.

As $n \ne 4$, we have that $p \ne 2$.

Hence $p > 2$.

Hence:
 * $2 p < p^2$

and so:
 * $2 p \divides \paren {n - 1}!$

By definition of factorial:
 * $\paren {n - 1}! = 1 \times 2 \times \ldots \times p \times \ldots \times 2 p \times \ldots \times \paren {n - 2} \times \paren {n - 1}$

and so:
 * $n = p^2 \divides \paren {n - 1}!$

Hence the result.