1. Painlevé, P.: Bull. Soc. Math. Fr.28, 214 (1900)
2. Painlevé, P.: Acta Math.25, 1 (1902)
3. Gambier, B.: Acta Math.33, 1 (1909)
4. Ince, E.L.: Ordinary differential equations (1927). New York: Dover 1956
5. The work done in Soviet Union up to 1976 is reviewed by the editor of Diff. Urav. in the following two review articles (where he quotes 127 references):