@article{MOMA_1997__124_4_178785,
     author = {J.S. Cassel and Zhanyuan Hou},
     title = {Initial {Value} {Problem} for {Mixed-Type} {Differential} {Equations.}},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {133--146},
     publisher = {mathdoc},
     volume = {124},
     number = {4},
     year = {1997},
     zbl = {0884.34073},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1997__124_4_178785/}
}