If , may not even be defined! (As we may choose to be )Originally Posted byummmm

Or have I missed something.

Of course I have missed something:

choose to be ,

then:

-the empty set.

Back to the problem:

(we need only consider the case where as the result holds trivally otherwise)

,

so if then and

and ,

i.e.

Now suppose then clearly

and so

hence:

RonL