@article{RSMUP_1994__91__65_0,
     author = {D'Ancona, Piero},
     title = {Well posedness in $C^\infty $ for a weakly hyperbolic second order equation},
     journal = {Rendiconti del Seminario Matematico della Universit\`a di Padova},
     pages = {65--83},
     publisher = {Seminario Matematico of the University of Padua},
     volume = {91},
     year = {1994},
     zbl = {0820.35097},
     language = {en},
     url = {http://www.numdam.org/item/RSMUP_1994__91__65_0/}
}