1. Beals, R. and Tenenblat, K., An intrinsic generalization for the wave and sine-Gordon equation,Differential Geometry, Pitman Monograph 52, Longman, Harlow, 1991, pp. 25?46.
2. Tenenblat, K.,A Note on Solutions for the Intrinsic Generalized Wave and Sine-Gordon Equation, Academic Press, New York.
3. Matveev, V. B. and Salle, M. A.,Darboux Transformations and Soliton, Springer-Verlag, New York, 1990.
4. Gu, Chaohao, Darboux transformation for a class of integrable systems inn variables, IDMF preprint 9221 (to appear in theProc. Conference in Honor of Y. Choquet-Bruhat).
5. Gu Chaohao, On the interaction of soliton for a class of integrable system in the spacetimeR n+1,Lett. Math. Phys. 26, 199?209 (1992).