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Topic: Using classes instead of sets
Replies: 26   Last Post: Apr 1, 2013 8:04 PM

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Frederick Williams

Posts: 2,164
Registered: 10/4/10
Re: Using classes instead of sets
Posted: Mar 31, 2013 7:49 AM
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"Shmuel (Seymour J.) Metz" wrote:
>
> In <g1udl8537unb3ue0drmnq1op9lj82r21la@4ax.com>, on 03/30/2013
> at 08:35 AM, David C. Ullrich <ullrich@math.okstate.edu> said:
>

> >Oh? For example?
>
> <http://en.wikipedia.org/wiki/New_Foundations>
>
> Note that it has only a restricted axiom of comprehension.


David Ullrich might be interested to know that NF proves infinity and
disproves choice! NF with Urelemente (and thus with Extensionality
relaxed) does neither. (Assuming NF/NFU to be consistent, natr\"ulich.
I think NFU is consistent iff PA is, and the consistency of NF is open.)

--
When a true genius appears in the world, you may know him by
this sign, that the dunces are all in confederacy against him.
Jonathan Swift: Thoughts on Various Subjects, Moral and Diverting



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