“…The problem treated in this paper belongs to a class of semiclassical inverse spectral questions which has attracted much attention in recent years, e.g. [21,24,25,15,29,34,39], which goes back to pioneer works of Bérard [1], Brüning-Heintze [3], Colin de Verdière [12,13], Duistermaat-Guillemin [19], and Guillemin-Sternberg [22], in the 1970s/1980s, and are closely related to inverse problems that are not directly semiclassical but do use similar microlocal techniques for some integrable systems, as in [40] (see also [41] and references therein).…”