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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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    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.2012-11-30

1 Answers 1

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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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    @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.2012-11-30