Let p be some prime divisor of the number 22n+1. Consider numbers x of the form 2k. By the statement of the problem we need xx−1=22s−1≡0(modp). If s≥n+1 then
222k−1=(22n+1)22k−(n+1)−1:22n+1−1=(22k−1)(22k+1),
where a:p denotes "a is divisible by p",i.e.p|a.Thus2^{2^k} - 1isdivisiblebyp.Soifallthenumbers2^{2^{n+1}}, 2^{2^{n+2}}, \dots, 2^{2^{2008}}arelessthanp,theproblemissolved.Thesolutionoftheproblemfollowsfromthelemma,statedbelow,forA = 2^{2008}$.
Lemma 1. For any positive integer number A there exists such n0 that all numbers of a kind 22n+1, where n≥n0, have prime divisor greater than A⋅22n.
To prove lemma 1, we need an additional lemma.
Lemma 2. If the prime number p is a divisor of the number 22n+1, then 2n+1≤p−1.
Proof (of lemma 2). Let Δ be the least positive integer such that p∣2Δ−1. We show that Δ∣p−1. It follows from Fermat's (little) theorem that 2p−1≡1(modp). Let Δ1 be the remainder in division of p−1 by Δ. Then from 2Δ≡1(modp) and 2p−1≡1(modp) it follows that 2Δ1≡1(modp). From the supposition of minimality of Δ it follows that Δ1=0, i.e. Δ∣p−1.
22n+1−1=(22n−1)(22n+1), so p∣22n+1. Reasoning by analogy with the above, one gets Δ∣2n+1, that is Δ=2k, k≤n+1. If k≤n then
22n+1=(22k)2n−2k+1≡12n−2k+1≡2(modp), which is impossible due to p∣22n+1. Hence Δ=2n+1, which (with Δ∣p−1 proven above) finishes the proof of the lemma.
Proof (of lemma 1). It follows from lemma 2 that all prime divisors of the number 22n+1 can be expressed as 2n+1βj+1. Let
22n+1=p1α1⋯psαs, where pj=2n+1βj+1.
Then
pjαj=(2n+1βj+1)αj=1+(1αj)2n+1βj+(2αj)22(n+1)βj2+⋯≡1+αj2n+1βj(mod22(n+1)).
Thus,
p1α1⋯psαs≡j=1∏s(1+αj2n+1βj)≡1+2n+1j=1∑sαjβj(mod22(n+1)).(1)
If 22n+2<22n (which is true for n>2), then 22n+1≡1(mod22(n+1)), hence we can get from (1) 2n+1∑j=1sαjβj≡0(mod22(n+1)), therefore ∑j=1sαjβj≡0(mod2n+1). Note that αj>0 and βj>0, so ∑j=1sαjβj≥2n+1.
Suppose that all βj<A (otherwise, when one of the βj≥A, let it be βj0, the number 22n+1 has a prime divisor 2n+1βj0+1>2n+1A and the lemma is proved). Then
2n+1≤j=1∑sαjβj<Aj=1∑sαj≤Asαk, where αk=1≤j≤smaxαj.
All pj are distinct numbers, thus all βj are also distinct numbers, and the number of βj is s. By supposition all βj<A, whence we get s<A. So 2n+1<A2αk, that is αk>A22n+1, but then pkαk=(2n+1βk+1)αk>(2n+1)αk>2(n+1)2n+1/A2. Since pkαk≤22n+1, then 2(n+1)2n+1/A2<22n+1. (2)
There exists such n0 that for n≥n0 we have A2n+1>1. Then it follows from (2) that
22n+1≥2(n+1)2n−1/A2>22n+1>22n+1.
a contradiction, which proves the lemma.