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26 elements of {1,2,3,.. ,40} such that product of 2of them is never square

Source: Mexican Mathematical Olympiad 1995 OMM P4

July 28, 2018
number theoryPerfect Square

Problem Statement

Find 2626 elements of {1,2,3,...,40}\{1, 2, 3, ... , 40\} such that the product of two of them is never a square. Show that one cannot find 2727 such elements.