Maths Olympiad Prep

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Number theory Difficulty 6.1 National olympiad Prove it

Example 4 Let pp be a prime. Prove: There exist infinitely many positive integers nn, such that p2nnp \mid 2^{n}-n.

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Solution

Prove that if p=2p=2, then taking nn as an even number, we have p2nnp \mid 2^{n}-n, and the proposition holds. Suppose p>2p>2, then by Fermat's Little Theorem, we know
2p11(modp)2^{p-1} \equiv 1(\bmod p)

Therefore, for any positive integer kk, we have
2k(p1)1(modp)2^{k(p-1)} \equiv 1(\bmod p)

So, it suffices to prove that there are infinitely many positive integers kk such that
k(p1)1(modp) (so, let n=k(p1), then p2nn)\left.k(p-1) \equiv 1(\bmod p) \text { (so, let } n=k(p-1) \text {, then } p \mid 2^{n}-n\right) \text {. }

And this only requires k1(modp)k \equiv-1(\bmod p), and there are certainly infinitely many such kk.
Therefore, the proposition holds.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.