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I am trying to prove that ($\forall \ n\in\mathbb{N}$) there exists a prime number $q$ such that $n < q \le 1 + n!$

I have made a graph with $n=0$ through $n=10$ and found solutions to all of them looking for a pattern and I see that $n!$ gets enormous fast and it becomes quite obvious that there is a prime number in between them.

I have considered trying to prove by contradiction that $q$ does not exist on that interval, but I don't know where to go from that statement. Could anybody help me figure it out? I have been staring at it for hours and I can't figure out where to go.

Thank you.

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6 Answers 6

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Hint: $n!+1$ has some prime factor $p$. If $p \leq n$ then $p\mid n!$.

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  • $\begingroup$ wow.. nice answer +1. Your answe is better than my one. $\endgroup$
    – Bumblebee
    Sep 26, 2014 at 4:25
  • $\begingroup$ I am looking at this, and I do not know what math to use. I cannot figure out what the first step would be, do I use a contradiction? then what do I do after I am stating a contradiction? Is it that since n! and 1+n! are not divisible then there must be a prime number p that is greater than n that is a factor of n!+1? I'm sorry I did not mean to enter the comment before I said that $\endgroup$
    – Sam
    Sep 26, 2014 at 4:36
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    $\begingroup$ @Sam You'll have to figure this out yourself. $\endgroup$ Sep 26, 2014 at 4:38
  • $\begingroup$ I'm not clear about something, maybe it's just me. The requirement was that we find a prime $p \ni \exists n \in \mathbb{Z^+}, n < p < n! + 1$. However, then we may not take $p \leq n.$ $\endgroup$ Sep 26, 2014 at 4:44
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    $\begingroup$ @ThisIsNotAnId You too will have to figure this out yourself. $\endgroup$ Sep 26, 2014 at 4:45
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HINT:
Use the Bertrand's postulate.
Since $n!\ge 2n$ for all $n\ge 3$ we have the result.

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    $\begingroup$ This is overkill. $\endgroup$ Sep 26, 2014 at 4:21
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    $\begingroup$ If by "large $n$" you mean "$n > 3$", then, sure.. $\endgroup$
    – Thomas
    Sep 26, 2014 at 14:14
  • $\begingroup$ The OP was looking for a way to prove the statement from scratch, I guess. $\endgroup$
    – user65203
    Nov 29, 2016 at 18:52
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All the primes dividing $n!$ give remainder $1$ when they divide $n!+1$. Those include all primes from $1$ to $n$. So either $n!+1$ is itself a prime, or it is divisible by a prime $>n$ and of course $\le n!+1$.

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For $n=1$ and $n=2$ the condition holds.

So to prove the statement assume $n>2$. Now for every integer $x$ such that $1<x<(n+1),$ we have $x|n!$ and $x\not|(n!-1).$

$\therefore$ either $(n!-1)$ is a prime, or $\exists$ a prime $p\ge (n+1) $ such that $p|(n!-1)$.

So in any case, $\exists$ a prime $p$ such that $(n+1)\le p\le (n!-1)$.

$\therefore$ $\exists$ a prime number $p$ such that $n<p≤1+n!$ $\hspace{.2cm}$$,\forall n\in \mathbb{N}.$

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$n!+1$ isn't divisible by the integers from $2$ to $n$. Then all its prime factors exceed $n$, and there is at least one.

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Case 1) if n=1,2,3 then $q=2,3,7$

Case 2) for other n, $2n< n! $ Now we know $\exists q$ prime s.t $n<q<2n<1+n!$

Hence we are done.

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    $\begingroup$ This is precisely Nilan's answer. $\endgroup$
    – daniel
    Sep 26, 2014 at 4:32
  • $\begingroup$ Yeah I have just seen. $\endgroup$
    – Ri-Li
    Sep 26, 2014 at 4:32

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