Hi again, Daniel cannot sleep tonight - perhaps from the feeling to loose too much time with these things. Yes - it's the wheel. And a dlist made from [3,5,7,11,13,17] was optimal in some of my experiments too. You will probably know it - but perhaps there are third-party readers: A last try to improve the "final" version was to replace Integer by Int64 (importing Prelude qualified). There was no difference ! That is good news and bad - the good news (and that's much more important), that Integer is absoulutely cleanly implemented. However, if this is so, i have little sympathy for being forced to use fromInteger and toInteger - for this special case i would prefer automatic coercion. And - sorry - cannot do other than seeing anything else as ill-advised dogmatism. But perhaps i am spoiled from Python :). I did this, because for secure decision about divisibility the program isn't useful beyond that. Thanks for that entry point: I took "google Rabin-Miller" in a comment of this last version. Happy days, Joost