@incollection{JEDP_1998____A10_0,
     author = {Nishitani, Tatsuo},
     title = {Hyperbolicity of two by two systems with two independent variables},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {10},
     pages = {1--12},
     publisher = {Universit\'e de Nantes},
     year = {1998},
     mrnumber = {2000k:35004},
     zbl = {01808719},
     language = {en},
     url = {http://www.numdam.org/item/JEDP_1998____A10_0/}
}