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Problem 1719

National Olympiad, first round
Number theory Difficulty 6.1 Prove it Iranian Mathematical Olympiad · Iran

Prove that for each positive integer mm, one can find mm consecutive positive integers like nn such that the following expression is not a perfect power
(13+20183)(23+20183)⋯(n3+20183) (1^3 + 2018^3)(2^3 + 2018^3)\cdots(n^3 + 2018^3)

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

Let pp be a prime number of the form 3k+23k+2, such that p>max⁡(m,2018)p > \max(m, 2018). (Such pp exists because there are infinitely many prime numbers of the form 3k+23k+2.) We shall prove that n=p−2019+in = p - 2019 + i, 1≤i≤m1 \le i \le m satisfies the problem's conditions.

It suffices to prove that
vp((13+20183)(23+20183)…(n3+20183))=1, v_p((1^3 + 2018^3)(2^3 + 2018^3)\dots(n^3 + 2018^3)) = 1,
as it would result in (13+20183)(23+20183)…(n3+20183)(1^3 + 2018^3)(2^3 + 2018^3)\dots(n^3 + 2018^3) not being a perfect power.
vp((13+20183)(23+20183)…(n3+20183))=vp(13+20183)+vp(23+20183)+⋯+vp(n3+20183). v_p((1^3 + 2018^3)(2^3 + 2018^3)\dots(n^3 + 2018^3)) = v_p(1^3 + 2018^3) + v_p(2^3 + 2018^3) + \dots + v_p(n^3 + 2018^3).
However, if ii exists such that i≠p−2018i \ne p - 2018 and vp(i3+20183)>0v_p(i^3 + 2018^3) > 0,
p∣i3+20183  ⟹  i3≡p(−2018)3  ⟹  (i3)p−23≡p((−2018)3)p−23  ⟹  (−2018)ip−1≡p(−2018)p−1i  ⟹  (−2018)≡pi  ⟹  p∣i+2018, i≠p−2018  ⟹  2p−2019>p−2019+m≥i≥2p−2018 \begin{align*} p|i^3 + 2018^3 &\implies i^3 \stackrel{p}{\equiv} (-2018)^3 \\ &\implies (i^3)^{\frac{p-2}{3}} \stackrel{p}{\equiv} ((-2018)^3)^{\frac{p-2}{3}} \\ &\implies (-2018)i^{p-1} \stackrel{p}{\equiv} (-2018)^{p-1}i \\ &\implies (-2018) \stackrel{p}{\equiv} i \implies p|i + 2018, \ i \ne p - 2018 \\ &\implies 2p - 2019 > p - 2019 + m \ge i \ge 2p - 2018 \end{align*}
which is a contradiction. Therefore,
vp((13+20183)(23+20183)…(n3+20183))=vp((p−2018)3+20183)=vp(p)+vp(3)=1 v_p((1^3 + 2018^3)(2^3 + 2018^3)\dots(n^3 + 2018^3)) = v_p((p - 2018)^3 + 2018^3) = v_p(p) + v_p(3) = 1
Due to L.T.E., which proves the claim. ■

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