[1] Université Paris 7,
[2] UFR de Mathématiques,undefined
[3] Case 7012,undefined
[4] 2,undefined
[5] place Jussieu,undefined
[6] F-75251 Paris Cedex 05,undefined
[7] France. e-mail: comets@math.jussieu.fr. Research partially supported by CNRS,undefined
[8] UMR 7599. Part of this work was done while visiting the Dept. of Electrical Eng.,undefined
[9] Technion,undefined
[10] Israel.,undefined
[11] Department of Mathematics,undefined
[12] TU Berlin. Strasse des 17. Juni 135,undefined
[13] D-10623 Berlin,undefined
[14] GERMANY. e-mail: gantert@math.TU-Berlin.DE. Research supported by the Swiss National Science foundation under grant 8220-046518,undefined
[15] and partially supported by the DFG. Part of this work was done while visting the Dept. of Electrical Eng.,undefined
[16] Technion,undefined
[17] Israel.,undefined
[18] Department of Electrical Engineering,undefined
[19] Technion-Israel Institute of Technology,undefined
[20] Haifa 32000,undefined
[21] ISRAEL. e-mail: zeitouni@ee.technion.ac.il. Partially supported by a US-Israel BSF grant and by IHP.,undefined