# [FOM] A corrcetion of a stupid mistake

Arnon Avron aa at tau.ac.il
Tue Sep 11 02:01:49 EDT 2007

Unfortunately, I was careless in my posting of Sun,  September 9 2007,
and made a really silly mistake of the type I would not
forgive my students for: to show the existence of
{{n}: n\in N} one needs of course only one application of the powerset
axiom, not two. This set *belongs* to P(P(N)) but its definition
in terms of separation and powerset is obviously just {x\in P(N): ...}.
The same goes with my other examples at that posting (thanks to
Piotr Rudnicki who took care to notify me).

I should note that embarrassing as it is, this  mistake  does not make
any difference for the thesis and content of that posting
(At worst I should have used, e.g. {{{n}}: n\in N} instead of
{{n}: n\in N}...).

Arnon Avron
School of Computer Science
Tel-Aviv University