I was looking at the resolution of the issue on this link (Find all positive integers $n$ such that $n$ divides $a^{25} - a$ for all positive integers $a$), but I have been trying for hours to understand why testing for 2 and 3 already guarantees the result for all "a" integers.
I had to make another post because I can't comment on posts yet. I can delete the post once my question is over if you want.
Thanks in advance