@article{10_4064_ap_50_2_191_208,
     author = {Jan Niechwiej},
     title = {Cosine families of unbounded normal operators},
     journal = {Annales Polonici Mathematici},
     pages = {191--208},
     publisher = {mathdoc},
     volume = {50},
     number = {2},
     year = {1989},
     doi = {10.4064/ap-50-2-191-208},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-50-2-191-208/}
}