On arithmetic progressions in nullspaces of integer matrices

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Inspired by the Erdös-Turan conjecture we consider subsets of the natural numbers that contains infinitely many aritmetic progressions (APs) of any given length - such sets will be called AP-sets and we know due to the Green-Tao Theorem and Sźmeredis Theorem that the primes and all subsets of positive upper density are AP-sets. We prove that (1, 1,..., 1) is a solution to the equation Mx = 0 where M is an integer matrix whose null space has dimension at least 2, if and only if the equation has infinitely many solutions such that the coordinates of each solution are elements in the same AP. This gives us a new arithmetic characterization of AP-sets, namely that they are the sets that have infinitely many solutions to a homogeneous system of linear equations, whenever the sum of the columns is zero.