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Number theory Difficulty 6.1 National Olympiad Prove it 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)

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=p2019+in = p - 2019 + i, 1im1 \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 ip2018i \ne p - 2018 and vp(i3+20183)>0v_p(i^3 + 2018^3) > 0,
pi3+20183    i3p(2018)3    (i3)p23p((2018)3)p23    (2018)ip1p(2018)p1i    (2018)pi    pi+2018, ip2018    2p2019>p2019+mi2p2018 \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((p2018)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. ■

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