What is the "x"?
Is it intersection? union?
Are A and B groups, sets, rings?
the x stands for multiplication.
If A multiply B = empty set then A = empty set or B = empty set.
or - stands for Union but in this question i don't think the "or" means Union in this question though.
although that's actually a very good question! hmmm..
If A multiply B = empty set then A = empty set or B = empty set.
No it does not stand for multiplication in a set theoretic context. It is a cross product: . Now it is perfectly clear that if either set is empty then the cross product is empty and visa versa.