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Number theory Difficulty 6.7 National olympiad Prove it Ukraine

Prove that there exist infinitely many such prime numbers pp that amongst {0,1,2,,p1}\{0,1,2,\dots, p-1\} can be found at least 2008 numbers xx such that xa1(modp)x^a \equiv 1 \pmod{p}.

Solution

Let pp be some prime divisor of the number 22n+12^{2^n} + 1. Consider numbers xx of the form 2k2^k. By the statement of the problem we need xx1=22s10(modp)x^x - 1 = 2^{2^s} - 1 \equiv 0 \pmod{p}. If sn+1s \ge n+1 then
222k1=(22n+1)22k(n+1)1:22n+11=(22k1)(22k+1), 2^{2^{2^k}} - 1 = (2^{2^{n+1}})^{2^{2^k-(n+1)}} - 1 : 2^{2^{n+1}} - 1 = (2^{2^k} - 1)(2^{2^k} + 1),
where a:pa: p denotes "a is divisible by p",i.e.p", i.e. p|a.Thus. Thus 2^{2^k} - 1isdivisibleby is divisible by p.Soifallthenumbers. So if all the numbers 2^{2^{n+1}}, 2^{2^{n+2}}, \dots, 2^{2^{2008}}arelessthan are less than p,theproblemissolved.Thesolutionoftheproblemfollowsfromthelemma,statedbelow,for, the problem is solved. The solution of the problem follows from the lemma, stated below, for A = 2^{2008}$.
Lemma 1. For any positive integer number AA there exists such n0n_0 that all numbers of a kind 22n+12^{2^n} + 1, where nn0n \ge n_0, have prime divisor greater than A22nA \cdot 2^{2^n}.
To prove lemma 1, we need an additional lemma.
Lemma 2. If the prime number pp is a divisor of the number 22n+12^{2^n} + 1, then 2n+1p12^{n+1} \le p-1.
Proof (of lemma 2). Let Δ\Delta be the least positive integer such that p2Δ1p|2^\Delta - 1. We show that Δp1\Delta | p-1. It follows from Fermat's (little) theorem that 2p11(modp)2^{p-1} \equiv 1 \pmod{p}. Let Δ1\Delta_1 be the remainder in division of p1p-1 by Δ\Delta. Then from 2Δ1(modp)2^\Delta \equiv 1 \pmod{p} and 2p11(modp)2^{p-1} \equiv 1 \pmod{p} it follows that 2Δ11(modp)2^{\Delta_1} \equiv 1 \pmod{p}. From the supposition of minimality of Δ\Delta it follows that Δ1=0\Delta_1 = 0, i.e. Δp1\Delta | p-1.

22n+11=(22n1)(22n+1)2^{2^{n+1}} - 1 = (2^{2^n} - 1)(2^{2^n} + 1), so p22n+1p \mid 2^{2^{n+1}}. Reasoning by analogy with the above, one gets Δ2n+1\Delta \mid 2^{n+1}, that is Δ=2k\Delta = 2^k, kn+1k \le n+1. If knk \le n then
22n+1=(22k)2n2k+112n2k+12(modp)2^{2^n} + 1 = (2^{2^k})^{2^n-2^k} + 1 \equiv 1^{2^n-2^k} + 1 \equiv 2 \pmod p, which is impossible due to p22n+1p \mid 2^{2^{n+1}}. Hence Δ=2n+1\Delta = 2^{n+1}, which (with Δp1\Delta \mid 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+12^{2^n} + 1 can be expressed as 2n+1βj+12^{n+1} \beta_j + 1. Let
22n+1=p1α1psαs, where pj=2n+1βj+1. 2^{2^n} + 1 = p_1^{\alpha_1} \cdots p_s^{\alpha_s}, \text{ where } p_j = 2^{n+1} \beta_j + 1.
Then
pjαj=(2n+1βj+1)αj=1+(αj1)2n+1βj+(αj2)22(n+1)βj2+1+αj2n+1βj(mod22(n+1)). p_j^{\alpha_j} = (2^{n+1} \beta_j + 1)^{\alpha_j} = 1 + \binom{\alpha_j}{1} 2^{n+1} \beta_j + \binom{\alpha_j}{2} 2^{2(n+1)} \beta_j^2 + \dots \equiv 1 + \alpha_j 2^{n+1} \beta_j \pmod{2^{2(n+1)}}.
Thus,
p1α1psαsj=1s(1+αj2n+1βj)1+2n+1j=1sαjβj(mod22(n+1)).(1) p_1^{\alpha_1} \cdots p_s^{\alpha_s} \equiv \prod_{j=1}^s (1 + \alpha_j 2^{n+1} \beta_j) \equiv 1 + 2^{n+1} \sum_{j=1}^s \alpha_j \beta_j \pmod{2^{2(n+1)}}. \quad (1)
If 22n+2<22n2^{2n+2} < 2^{2^n} (which is true for n>2n > 2), then 22n+11(mod22(n+1))2^{2^n} + 1 \equiv 1 \pmod{2^{2(n+1)}}, hence we can get from (1) 2n+1j=1sαjβj0(mod22(n+1))2^{n+1} \sum_{j=1}^s \alpha_j \beta_j \equiv 0 \pmod{2^{2(n+1)}}, therefore j=1sαjβj0(mod2n+1)\sum_{j=1}^s \alpha_j \beta_j \equiv 0 \pmod{2^{n+1}}. Note that αj>0\alpha_j > 0 and βj>0\beta_j > 0, so j=1sαjβj2n+1\sum_{j=1}^s \alpha_j \beta_j \ge 2^{n+1}.
Suppose that all βj<A\beta_j < A (otherwise, when one of the βjA\beta_j \ge A, let it be βj0\beta_{j_0}, the number 22n+12^{2^n} + 1 has a prime divisor 2n+1βj0+1>2n+1A2^{n+1} \beta_{j_0} + 1 > 2^{n+1} A and the lemma is proved). Then
2n+1j=1sαjβj<Aj=1sαjAsαk, where αk=max1jsαj. 2^{n+1} \le \sum_{j=1}^s \alpha_j \beta_j < A \sum_{j=1}^s \alpha_j \le A s \alpha_k, \text{ where } \alpha_k = \max_{1 \le j \le s} \alpha_j.
All pjp_j are distinct numbers, thus all βj\beta_j are also distinct numbers, and the number of βj\beta_j is ss. By supposition all βj<A\beta_j < A, whence we get s<As < A. So 2n+1<A2αk2^{n+1} < A^2 \alpha_k, that is αk>2n+1A2\alpha_k > \frac{2^{n+1}}{A^2}, but then pkαk=(2n+1βk+1)αk>(2n+1)αk>2(n+1)2n+1/A2p_k^{\alpha_k} = (2^{n+1} \beta_k + 1)^{\alpha_k} > (2^{n+1})^{\alpha_k} > 2^{(n+1)2^{n+1}/A^2}. Since pkαk22n+1p_k^{\alpha_k} \le 2^{2^n} + 1, then 2(n+1)2n+1/A2<22n+12^{(n+1)2^{n+1}/A^2} < 2^{2^n} + 1. (2)
There exists such n0n_0 that for nn0n \ge n_0 we have n+1A2>1\frac{n+1}{A^2} > 1. Then it follows from (2) that
22n+12(n+1)2n1/A2>22n+1>22n+1. 2^{2^n} + 1 \ge 2^{(n+1)2^{n-1}/A^2} > 2^{2^{n+1}} > 2^{2^n} + 1.
a contradiction, which proves the lemma.

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