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NMO Selection Tests for BMO and IMO

Number theory Difficulty 7.9 National Olympiad, round 2 Prove it Romania

Let kk be a positive integer and let mm be a positive odd integer. Show that there exists a positive integer nn such that mn+nmm^n + n^m has at least kk distinct prime factors.

Solution

Design a set of kk primes p1<p2<⋯<pkp_1 < p_2 < \cdots < p_k as follows. Begin by choosing p1>2mp_1 > 2m. Having selected pjp_j, use Dirichlet's theorem to choose a prime
pj+1≡−1(modp1(p1−1)p2(p2−1)⋯pj(pj−1)). p_{j+1} \equiv -1 \pmod{p_1(p_1-1)p_2(p_2-1)\cdots p_j(p_j-1)}.
If i<ji < j, then pi<pjp_i < p_j, so pjp_j does not divide pi−1p_i - 1; further, pj−1≡−2(modpi(pi−1))p_j - 1 \equiv -2 \pmod{p_i(p_i - 1)}, so pip_i does not divide pj−1p_j - 1.

Next, use the Chinese Remainder Theorem, to choose a positive integer nn such that n≡−1(modp1p2⋯pk)n \equiv -1 \pmod{p_1p_2\cdots p_k} and n≡0(mod(p1−1)⋯(pk−1))n \equiv 0 \pmod{(p_1-1)\cdots(p_k-1)}.

Finally, since p1p2⋯pkp_1p_2\cdots p_k and mm are coprime, and (p1−1)(p2−1)⋯(pk−1)(p_1-1)(p_2-1)\cdots(p_k-1) divides nn, and mm is odd, Euler's Theorem applies to show that mn+nm≡1+(−1)m≡0(modp1p2⋯pk)m^n + n^m \equiv 1 + (-1)^m \equiv 0 \pmod{p_1p_2\cdots p_k}. The conclusion follows.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.