@article{IM_1990__100_2_143785,
     author = {H. Kn\"orrer and J. Feldman and E. Trubowitz},
     title = {The pertubatively stable spectrum of a periodic {Schr\"odinger} operator.},
     journal = {Inventiones mathematicae},
     pages = {259--300},
     publisher = {mathdoc},
     volume = {100},
     number = {2},
     year = {1990},
     zbl = {0701.34082},
     url = {http://geodesic.mathdoc.fr/item/IM_1990__100_2_143785/}
}