The largest integer that divides $p^4-1$, in which p is a prime greater than $5$ The question is to find the largest integer that divides all $p^4-1$, where p is a prime greater than 5. Being asked this question, I just assume this number exists. Set $p = 7$, then $p^4-1=2400$. I don't have any background in number theory and not sure what to do next. Thank you for your help! 
 A: Let $n$ be the largest integer that divides $p^4-1$ for all prime $p\geq 7.$ 
We have $11^4-1=14640$ and $7^4-1=2400.$ The $gcd$ of $14640$ and $2400$ is $240.$ So  $$n\leq 240.$$ If $p$ is odd then modulo $16$ we have $p^4\in \{(\pm 1)^4, (\pm 3)^4,(\pm 5)^4,(\pm 7)^4\}=\{1^2,9^2, 25^2, 49^2\}=\{1^2,9^2,9^2,1^2\}=$
 $=\{1,81,81,1\}=\{1\}.$  
If $p$ is not divisible by $3$ then modulo $3$ we have $p^4\in \{(\pm 1)^4\}=\{1\}.$
If $p$ is not divisible by $5$ then modulo $5$ we have $p^4\in \{(\pm 1)^4,(\pm 2)^4\}=\{1,16\}=\{1\}.$
So for any integer $p$ that is not divisible by $2,3,$ or $5$  we have $p^4\equiv 1 \pmod {16}$ and $p^4  \equiv 1 \pmod 3$ and $p^4 \equiv 1 \pmod 5;$ and  since $16,3,$ and 5 are pair-wise co-prime, therefore  $p^4\equiv 1 \pmod {16\cdot 3\cdot 5}=240,$ so $$n\geq 240.$$
A: Let $n$ be the number you are trying to find.
One of the main reasons to look at an example $p=7$ is to gain information about $n$. If $n$ divides all $p^4 - 1$, and $7^4 - 1 = 2400$, then $n$ divides $2400$.
Thus, you've gained information about $n$. Follow that up — what information does knowing that $n \mid 2400$ give you?
In my opinion, the next obvious things to do are to try the next few primes and gain more information about $n$.
My expectation is that will be enough to completely determine what $n$ ought to be, or at least narrow it down to a very small number of possibilities, at which point you can turn towards trying to prove that one of the possibilities is actually the value of $n$.
A: Since $p^4-1$ is relatively prime to $p$, we know that the common divisor will not have any large prime factors.  Indeed, from your calculation, it will have to be a divisor of $2400=2^5*3*5^2$.  However, you could make an even better guess by trying out a few more primes and then taking the greatest common divisor.  However, that will get you a proof.
A reasonable thing to try is to factor the polynomial $p^4-1=(p^2+1)(p^2-1)=(p^2+1)(p+1)(p-1)$.  There are various things you can see from this.  For example, since $p$ is odd, $p\pm 1$ are both even, as is $p^2+1$, giving you at least three factors of $2$.  Actually, one of $p\pm 1$ has to be multiple of $4$ (being two consecutive even numbers), and therefore you know that $16$ will divide $p^4-1$ for all odd $p$ (even if $p$ isn't prime).  But you should gather more data to figure out exactly what you want to prove.
