Question

Question #e0335

Answer 1

This is because if the set `A` is empty, you can't put at the first position anything...

You can also see this, remembering that the cross product can be imagined like a rectangle in which in one side there are the elements of `A` and in the other side there are the elements of `B`.
The possible couples you can do are the area of that rectangle.
But if one side has lenght zero... the area is zero!