Can anyone explain why GAP behaves as follows? gap> Intersection([1..3],[1..3]); [ 1 .. 3 ] gap> Intersection([-3..-1],[-3..-1]); [ -3 .. -1 ] gap> Intersection([-1..1],[-1..1]); [ ] ____________________________________________________ Matthew Fayers Queen Mary, University of London Mile End Road London E1 4NS U.K.