# Find the biggest number $k$ such that $k| n^{55}-n$

Find the biggest integer number $k$ such that $k$ divides $n^{ 55 }-n$, where $n$ is any integer number.

This question was a challenge from my teacher, here's my attempt :

• because $\left( n \right) \left( n-1 \right) \left( { n }^{ 53 }+{ n }^{ 52 }+{ n }^{ 51 }+...+n+1 \right) \equiv 0 \pmod k$, therefore $n$ could be $2$,
but my teacher told me that there is a number which is bigger than $2$.

I've tried to solve it many times, but unfortunately I didn't get any solution. So I hope that you can help me to approach this problem.

• Did you really need to enclose all your text in dollar signs? – Sean Roberson Jan 27 '17 at 18:43
• Why man, why. I can't even edit it without considerable effort due to all the backslashes.. – s.harp Jan 27 '17 at 18:55
• Life is hard... – TheGeekGreek Jan 27 '17 at 18:56
• Why don't you try to similar things that worked here? – Jyrki Lahtonen Jan 27 '17 at 18:59
• @Jyrki But there is no need to factor any large numbers. It suffices to factor $54$ - see my answer. – Bill Dubuque Jan 27 '17 at 19:38

Hint $\$ By the Theorem below we deduce $\,k\mid n^{\large 55}-n\,$ for all $n$ iff $\,k\,$ is a product of distinct primes $p$ such that $\,p-1\mid 54$, i.e. $\,p =2,3,7,19.\,$ Thus the largest such $k$ is their product $= 798.$

Theorem $\$ For natural numbers $\rm\:a,e,n\:$ with $\rm\:e,n>1$

$\qquad\rm n\:|\:a^{\large e}-a\:$ for all $\rm\:a\:\iff n\:$ is squarefree, and prime $\rm\:p\:|\:n\:\Rightarrow\: p\!-\!1\:|\:e\!-\!1$

Proof $\ (\Leftarrow)\ \$ Since a squarefree natural divides another iff all its prime factors do, we need only show $\rm\:p\:|\:a^{\large e}\!-\!a\:$ for each prime $\rm\:p\:|\:n,\:$ or, that $\rm\:a \not\equiv 0\:\Rightarrow\: a^{\large e-1} \equiv 1\pmod p,\:$ which, since $\rm\:p\!-\!1\:|\:e\!-\!1,\:$ follows from $\rm\:a \not\equiv 0\:$ $\Rightarrow$ $\rm\: a^{\large p-1} \equiv 1 \pmod p,\:$ by little Fermat.
$(\Rightarrow)\ \$ See this answer.