1. T. Nishitani, Symmetrization of a class of hyperbolic systems with real constant coefficients, Ann. Scuola Norm. Sup. Pisa, Cl. sc. 21 (1994), 97–130.
2. T. Nishitani and J. Vaillant, Smoothly symmetrizable systems and the reduced dimensions, Tsukuba J. Math. 25 (2001) n. 1, 165–177.
3. T. Nishitani and J. Vaillant, Smoothly symmetrizable systems and the reduced dimensions II, Tsukuba J. Math. 27 (2003) n. 2, 389–403.
4. T. Nishitani and J. Vaillant, Smoothly symmetrizable complex systems and the real reduced dimension. Tsukuba J. Math. To appear.
5. G. Strang, On strong hyperbolicity, J. Math. Kyoto Univ. 6 (1967), 397–417.