1. Beals, M.: Self spreading and strength of singularities for solutions to semi-linear wave equations. Ann. Math.118, 187?214 (1983)
2. Beals, M.: Propagation of smoothness for non linear second order strictly hyperbolic differential equations. Proc. Symp. Pure Math.43, 21?44 (1985)
3. Bony, J-M.: Calcul symbolique et propagation des singularit�s pour les �quations aux d�riv�es partielles non lin�aires. Ann. Sci. Ec. Norm. Super., IV. Ser.14, 209?246 (1981)
4. Bony, J-M.: Interaction des singularit�s pour les �quations aux d�riv�es partielles non in�aires, S�m. Goulaouic-Meyer-Schwartz, 1981/82, n02 et Interaction pour Klein Gordon, S�m. Goulaouic-Meyer-Schwartz, 1 1983/84, n0 10
5. Advances in Microlocal Analysis, Castel Vecchio 1985, Nato Asi. Series.;J-M. Bony,1986