“…These properties are known as global hypoellipticity and global solvability, and have been widely studied in recent years, especially on the d−dimensional torus T d . See, for example, the impressive list of authors who have published articles addressing these subjects: [3], [4], [5], [6], [10], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [28], [29] and references therein.…”