# how to prove $437\,$ divides $18!+1$? (NBHM 2012)

I was solving some problems and I came across this problem. I didn't understand how to approach this problem. Can we solve this with out actually calculating $18!\,\,?$

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$18!+1$ does not divide $437$; do you mean that $437$ divides $18!+1$? – Brian M. Scott Nov 30 '12 at 7:15
thanks for the correction. – Phani Raj Nov 30 '12 at 7:17
i got upto that point. Now we have to prove that 23(because 437=19*23) also divides 18!+1. i am unable to prove that part. – Phani Raj Nov 30 '12 at 7:18

Note that $437=(19)(23)$. We prove that $19$ and $23$ divide $18!+1$. That is enough, since $19$ and $23$ are relatively prime.

The fact that $19$ divides $18!+1$ is immediate from Wilson's Theorem, which says that if $p$ is prime then $(p-1)!\equiv -1\pmod{p}$.

For $23$ we need to calculate a bit. We have $22!\equiv -1\pmod{23}$ by Wilson's Theorem.

Now $(18!)(19)(20)(21)(22)=22!$.

But $19\equiv -4\pmod{23}$, $20\equiv -3\pmod{23}$, and so on. So $(19)(20)(21)(22)\equiv 24\equiv 1\pmod{23}$. It follows that $18!\equiv 22!\pmod{23}$, and we are finished.

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Could you explain "relatively prime" in this context? 19 and 23 are prime - what's relative about that? – Tim Pietzcker Nov 30 '12 at 14:25
Two numbers a,b are relatively prime (or "coprime") iff gcd(a,b)=1, see en.wikipedia.org/wiki/Coprime_integers. – Landei Nov 30 '12 at 14:34
@TimPietzcker: Also, the significance in this context is that if a and b are relatively prime, and both divide c, then their product also divides c for hopefully obvious reasons. – camccann Nov 30 '12 at 15:10
@TimPietzcker The implication is still valid even if you were using, say, 12 and 35, even though those are not prime - it is enough that they not share any prime factors, which is what "relatively prime" means. – Thomas Nov 30 '12 at 15:11

That's not so big numbers. This is a demonstration: 18! + 1 = 6402373705728001 = 14650740745373 * 437

Of course that's not as interesting as André's one.

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The OP was asking to solve it without calculation $18!$. – martini Nov 30 '12 at 9:55