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Tying unit cubes to a thread and building a cuboid

Source: Bundeswettbewerb Mathematik 2024, Round 1 - Problem 4

March 8, 2024
combinatoricscombinatorics proposed

Problem Statement

For positive integers pp, qq and rr we are given pqrp \cdot q \cdot r unit cubes. We drill a hole along the space diagonal of each of these cubes and then tie them to a very thin thread of length pqr3p \cdot q \cdot r \cdot \sqrt{3} like a string of pearls.
We now want to construct a cuboid of side lengths pp, qq and rr out of the cubes, without tearing the thread. a) For which numbers pp, qq and rr is this possible? b) For which numbers pp, qq and rr is this possible in a way such that both ends of the thread coincide?