@incollection{JEDP_1989____A8_0,
     author = {Matsumoto, Waichiro and Yamahara, Hideo},
     title = {Necessary conditions for strong hyperbolicity of first order systems},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {8},
     pages = {1--16},
     publisher = {Ecole polytechnique},
     year = {1989},
     zbl = {0703.35100},
     mrnumber = {1030823},
     language = {en},
     url = {http://www.numdam.org/item/JEDP_1989____A8_0/}
}