Global Hypoellipticity for Perturbations of Complex Vector Fields on the Torus

Abstract We apply Krönecker’s approximation theorem to measure (in a topological sense) a set of constants which turn a vector field into a non-globally hypoelliptic operator. We present situations in which this set is a discrete enumerable (hence, meager) subset of the real line, and we also show that this set may be a dense $$\mathcal {G}_\delta $$ G δ subset of the complex numbers (hence, nonmeager), which produces a contrast to a known result stating that this set has null Lebesgue measure.