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Macedonia National Olympiad 2017 Problem 2

Source: Macedonia National Olympiad 2017

April 8, 2017
number theoryalgebra

Problem Statement

Find all natural integers nn such that (n3+39n2)n!+1721n+5(n^3 + 39n - 2)n! + 17\cdot 21^n + 5 is a square.