1. Benvenuti, S., Nishitani, T.: Necessary conditions for the hyperbolicity of 2 x 2 systems, Japanese J. Math. 25 (1999), 377 - 408.
2. Bove, A., Nishitani, T.: Necessary conditions for hyperbolic systems, in preparation.
3. Colombini, F., Nishitani, T.: Two by two strongly hyperbolic systems and the Gevrey classes. Ann. Univ. Ferrara Sc. Mat. Suppl. XLV (1999), 291 - 312.
4. D’Ancona, P., Spagnolo, S.: On pseudosymmetric hyperbolic systems. Ann. Scuola Norm. Sup. Pisa 25 (1997), 397 - 417.
5. Dieudonné, J.: Les déterminants sur un corps non commutatif. Bull. Soc. Math. France 71 (1943), 27 - 45.