olymmath

ISL 2020 N1

Given a positive integer kk, show that there exists a prime pp such that one can choose distinct integers a1,a2,…,ak+3∈{1,2,…,p−1}a_{1}, a_{2}, \ldots, a_{k+3} \in\{1,2, \ldots, p-1\} such that pp divides aiai+1ai+2ai+3−ia_{i} a_{i+1} a_{i+2} a_{i+3}-i for all i=1,2,…,ki=1,2, \ldots, k. (South Africa)

Official text Transcription from NuminaMath olympiads, checked word for word against the official paper. Quoted with source.

Official text: IMO 2020 Shortlist booklet, page 12.

Solutions

Official solution Solution 1 by the IMO Problem Selection Committee

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