Université Denis Diderot and LAN Université Pierre et Marie Curie Paris France. E-mail: bardos@math.jussieu.fr;
Division of Applied Mathematics Brown University Providence Rhode Island 02912 USA.E-mail: guoy@cfm.brown.edu;
Department of Mathematics Brown University Providence Rhode Island 02912 USA.E-mail: wstrauss@mat h.brown.edu;
Stable ideal plane flows; Unstable ideal plane flows; Liapunov exponent;