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