[1] Department of Applied Mathematics,
[2] Charles University,undefined
[3] 118 00 Prague 1
,undefined
[4] Department of Mathematics,undefined
[5] Case Western University,undefined
[6] Cleveland,undefined
[7] OH 44106,undefined
[8] Departament de Matematica Aplicada II,undefined
[9] Universitat Politecnica de Catalunya,undefined
[10] E-08034 Barcelona,undefined