Number theory · Divisibility · Residues modulo 4 · Decimal representation · Difference of squares
Prove that a positive integer whose decimal representation uses no digits other than \(2\) and \(6\) cannot be written as a difference of the squares of two integers.
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Serbian Regional Competition (Okruzno takmicenje) 1995, high school grade I, problem 1. The 1995 regional paper was not split into categories. Organized by the Mathematical Society of Serbia (DMS).