Hi, thanks a lot for your answers. I knew there is a method to solve my probelm, but forgot how to do it. Greetings, Uwe. -- Dr. Uwe Schmitt Uwe.Schmitt at num.uni-sb.de Universitaet des Saarlandes Angewandte Mathematik Building 36.1 Room 4.17 PO-Box 151150 D-66041 Saarbruecken Mobile:0177/6806587 Fax:+49(0)681/302-4435 Office:+49(0)681/302-2468