Proof
First, we prove a lemma: Let a be a positive integer greater than 1, and q be an odd prime factor of the number a2n+1, then
q≡1(mod2n+1)
In fact, from a2n≡−1(modq), we know
a2n+1≡1(modq),
This indicates that δq(a)∤2n, but δq(a)∣2n+1. Therefore, δq(a)=2n+1 (note that we have used q>2 here). Then, by Fermat's Little Theorem, we know aq−1≡1(modq), hence 2n+1∣(q−1), i.e.,
q≡1(mod2n+1)
The lemma is proved.
Returning to the original problem. When n>1, note that,
Fn−12n+1=(22n−1+1)2n+1=(22n+21+2n−1+1)2n≡(21+2n−1)2n=(22n)1+2n−1≡(−1)1+2n−1≡−1(modFn)
Therefore, for a prime factor q of Fn, we have q∣(Fn−122+1+1). Using the conclusion of the lemma, we get
q≡1(mod2n+2)