Cartesain product with the empty set is empty. A x B is the set of all pairs (a,b) where a is in A and b is in B. If either is empty then there are no such pairs and the result is also empty.

-- Mark Biggar [EMAIL PROTECTED] [EMAIL PROTECTED] [EMAIL PROTECTED] -------------- Original message ---------------------- From: TSa <[EMAIL PROTECTED]> > HaloO, > > why is (1,2,3) X () defined to be the empty list > and not (1,2,3) as is the case with the cartesian > product of sets which X basically is with preserved > order. > > Regards, TSa. > -- > > The Angel of Geometry and the Devil of Algebra fight for the soul > of any mathematical being. -- Attributed to Hermann Weyl