“…Inverse monodromy problems have recently been discussed in [4], [5], [6], [15], [68], [69], [96], [101], and the literature cited therein. More specific inverse spectral problems, such as compactness of the isospectral set of periodic Schrödinger operators [13], special isospectral matrix-valued Schrödinger operators, and Borg-type uniqueness theorems (for periodic coefficients as well as eigenvalue problems on compact intervals) were recently studied in [17], [18], [19], [21], [23], [26], [51], [52], [68], [69], [101], [102], [103], [104]. Moreover, direct spectral theory in the particular case of periodic Schrödinger operators (i.e., Floquet theory and the like) has been studied in [12], [14], [23], [25], [26], [34], [53], [59], [60], [86], [102], [103], [110]- [112], with many more pertinent references to be found therein.…”