1. See, for example, Encyklopädie d. math. Wissenschaften II A 7 [Sommerfeld], and R. d'Adhémar, Les équations aux dérivées partielles à caractéristiques réelles, Paris 1907. The work of Picard, in particular, has been of fundamental importance. See § 3 below.
2. First solved byPicard for the special case ?(x)=x. See Note I in Théorie générale des surfaces, 4, p. 353. More general problems of this nat recently formed the subject of investigations byGoursat. See Annales de Toulouse, 2e série. t. V and VI.
3. First rigorous solution was given byPicard. Note I inDarboux générale des surfaces, 4, p. 353.
4. Picard, Journal de Math. 4e série, 6 (1890).
5. Bulletin de la Soc. math. de France, 1900, 1903.