[FOM] A new definition of Cardinality.
Zuhair Abdul Ghafoor Al-Johar
zaljohar at yahoo.com
Mon Nov 30 19:36:39 EST 2009
Dear Mr. Forster:
You seem to be right regarding the concept of not every set is equinumerous to a well founded set. However the way how I see matters, is that every set MUST have a cardinality that is a set despite choice and regularity. We need to find a general definition of cardinality in ZF minus Regularity (and without choice).
However as a first step, Scott's trick is acceptable, for at least it defines cardinality for all well founded sets despite choice.
By the way, I have the following question in my mind, which might be related to Scott's trick?
We know from ZF that for every set x there exist a transitive closure
TC(x) that is a set.
Now does ZF(with Regularity of course)
prove or refute the following?
For all x Exist y
(y equinumerous to x and
not Exist z (z equinumerous to x and TC(z) strictly subnumerous to TC(y)))
x subnumerous to y <-> Exist f (f:x-->y, f is injective)
x equinumerous to y <-> Exist f (f:x-->y, f is bijective)
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