Sniffnoy comments on Stupid Questions Open Thread Round 2 - Less Wrong

15 Post author: OpenThreadGuy 20 April 2012 07:38PM

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Comment author: Sniffnoy 22 April 2012 08:32:24PM 1 point [-]

True, I forgot about that. I guess that would allow extending the notion of "computable well-ordering" to sets of size 2^aleph_0. Of course that doesn't necessarily mean any of them would actually be computable, and I expect none of them would. Well, I guess it has to be the case that none of them would, or else we ought to be able to prove "R is well-orderable" without choice, but there ought to be a simpler argument than that -- just as computable ordinals in the ordinary sense are downclosed, I would expect that these too ought to be downclosed, which would immediately imply that none of the uncountable ones are computable. Now I guess I should actually check that they are downclosed...

Comment author: JoshuaZ 22 April 2012 08:50:56PM 0 points [-]

What does it mean to be downclosed?

Comment author: Sniffnoy 22 April 2012 10:25:42PM 1 point [-]

I mean closed downwardly -- if one is computable, then so is any smaller one. (And so the Church-Kleene ordinal is the set of all computable ordinals.)