Hi, I seem to remember a Numerical function that could produce the list of values (or array positions) which are common to two arrays. In other words: a1=[1,2,3,9,10] a2=[1,4,9,10] result is [1,9,10]. Browsing through the Numerical docs doesn't refresh my memory, so any kind souls? many thanks, José -- José L Gómez Dans PhD student Tel: +44 114 222 5582 Radar & Communications Group FAX; +44 870 132 2990 Department of Electronic Engineering University of Sheffield UK