[FOM] A new definition of Cardinality.
meskew at math.uci.edu
Tue Nov 24 20:44:49 EST 2009
It is worth pointing out that your definition still has a disadvantage
if you don't assume choice. Without choice, not all cardinalities are
comparable. If they were then all cardinalities in your sense would
be comparable to a cardinality that contains a Von Neumann ordinal,
but from this you could derive choice. (It is nice to have a linear
order on set sizes.)
I'm not sure if you can prove in ZF that every set has a cardinality
in your sense. For every set X is there a set Y and a function f such
that Y is hereditary and f is a bijection between X and Y?
On Sun, Nov 22, 2009 at 2:16 PM, Zuhair Abdul Ghafoor Al-Johar
<zaljohar at yahoo.com> wrote:
> 1) Von Neumann's cardinals has the limitation of being dependent on
> choice, without choice one cannot know what is the cardinality of
> Power(omega) for example.Accordingly in any theory which do not have
> the axiom of choice among its axioms most of its sets would be of
> indeterminable cardinality, which is a big draw back.
> 4) The cardinality of any set x is: The class of all sets
> that are equinumerous to x were every member of their transitive
> closure is strictly subnumerous to x.
> So for any set x, any y is a member of the cardinality of x,
> if and only if, y is equinumerous to x and every member of the
> transitive closure of y is strictly subnumerous to x.
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