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A property of spheres defined on rational grids

Source: Romania National Olympiad 2016, grade ix, p. 3

August 25, 2019
number theorygeometry3D geometrysphere

Problem Statement

We say that a rational number is spheric if it is the sum of three squares of rational numbers (not necessarily distinct). Prove that:
a) 7 7 is not spheric. b) a rational spheric number raised to the power of any natural number greater than 1 1 is spheric.