@article{AIHPA_1989__50_3_309_0,
     author = {Howland, James S.},
     title = {Floquet operators with singular spectrum. {I}},
     journal = {Annales de l'I.H.P. Physique th\'eorique},
     pages = {309--323},
     publisher = {Gauthier-Villars},
     volume = {50},
     number = {3},
     year = {1989},
     mrnumber = {1017967},
     zbl = {0689.34022},
     language = {en},
     url = {http://www.numdam.org/item/AIHPA_1989__50_3_309_0/}
}