@article{CMH_1978__53_139759,
     author = {Toshikazu Sunada},
     title = {Spectrum of a compact flat manifold.},
     journal = {Commentarii mathematici Helvetici},
     pages = {613--621},
     year = {1978},
     volume = {53},
     zbl = {0422.53023},
     url = {http://geodesic.mathdoc.fr/item/CMH_1978__53_139759/}
}