olymmath

OMM 2021 Problem 5

    If n=a1a2⋯ak−1ak‾n=\overline{a_1a_2\cdots a_{k-1}a_k} , be s(n)s(n) such that . If kk is even, s(n)=a1a2‾+a3a4‾⋯+ak−1ak‾s(n)=\overline{a_1a_2}+\overline{a_3a_4}\cdots+\overline{a_{k-1}a_k} . If kk is odd, s(n)=a1+a2a3‾⋯+ak−1ak‾s(n)=a_1+\overline{a_2a_3}\cdots+\overline{a_{k-1}a_k} For example s(123)=1+23=24s(123)=1+23=24 and s(2021)=20+21=41s(2021)=20+21=41 Be nn is digitaldigital if s(n)s(n) is a divisor of nn . Prove that among any 198 consecutive positive integers, all of them less than 2000021 there is one of them that is digitaldigital .

    Dataset text AI-MO AoPS. Quoted with source.

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