@article{CMH_1990__65_3_140178,
     author = {Th. Kappeler},
     title = {On the periodic spectrum of the 1-dimensional {Schr\"odinger} operator.},
     journal = {Commentarii mathematici Helvetici},
     pages = {1--3},
     publisher = {mathdoc},
     volume = {65},
     number = {3},
     year = {1990},
     zbl = {0703.34085},
     url = {http://geodesic.mathdoc.fr/item/CMH_1990__65_3_140178/}
}