[1] Institut Fourier,
[2] URA 188 du CNRS,undefined
[3] Université de Grenoble 1,undefined
[4] BP 74,undefined
[5] F-38402 St. Martin d'Hères Cedex,undefined
[6] France ,undefined
[7] Department of Mathematics,undefined
[8] Box U-9,undefined
[9] The University of Connecticut,undefined
[10] Storrs,undefined
[11] CT 06269-3009,undefined
[12] U.S.A.,undefined