@article{CMH_1991__66_4_140248,
     author = {H. Kn\"orrer and E. Trubowitz and J. Feldmann},
     title = {Perturbatively unstable eigenvalues of a periodic {Schr\"odinger} operator.},
     journal = {Commentarii mathematici Helvetici},
     pages = {557--579},
     publisher = {mathdoc},
     volume = {66},
     number = {4},
     year = {1991},
     zbl = {0763.35024},
     url = {http://geodesic.mathdoc.fr/item/CMH_1991__66_4_140248/}
}