Topological methods for ordinary differential equations: lectures given at the 1st session of the Centro internazionale matematico estivo (C.I.M.E.), held in Montecatini Terme, Italy, June 24-July 2, 1991 - Fitzpatrick, Patrick,; Furi, M.; Zecca, P.; Furi, M; Zecca, P (Lecture notes in mathematics, 1993)