Re: Infinities

Nick Bostrom (
Mon, 17 Nov 1997 23:44:45 +0000

John K Clark wrote:

> If A is an infinite number then A -A = A not zero, A+A = A too, and A*A =A,
> but 2^A is not equal to A, it's a higher infinity.

Actually, it is not true that A-A=A for infinite cardinals A.
Sums involving positive and negative Cantorian infinities are not
defined. Robinsonian infinimal hyperreals do have addition defined
on them, but there I+(-I)=0. (It's because John's posts are usually
so accurate and correct that it is worth picking on small things like

Nick Bostrom