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Topic: This Week's Finds in Mathematical Physics (Week 236)
Replies: 29   Last Post: Aug 24, 2006 9:00 AM

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 john baez Posts: 460 Registered: 12/6/04
Re: This Week's Finds in Mathematical Physics (Week 236)
Posted: Aug 8, 2006 4:09 AM
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In article <1154931929.924105.82850@n13g2000cwa.googlegroups.com>,
Keith Ramsay <kramsay@aol.com> wrote:

>I don't think it's so hard to see that the way one ordinarily proves
>induction up to Gamma_0 is impredicative. It's not that it's impossible
>to define it predicatively. Each computable ordinal can be defined
>as an ordering on natural numbers, given by a primitive recursive
>relation on them. The existence of this ordering isn't the problem.
>The problem is proving induction up to it. The way that one ordinarily
>does it makes reference to sets of ordinals. That's the gist of it. To
>show that this is not a merely apparent obstacle to a predicative
>proof is a longer story.

I'd love to hear a bit more of the story, especially if you can tell
it in a charming and not too rigorous manner. In particular, nothing
in the paragraph says what's special about Gamma_0. For example,
suppose I have an ordinal smaller than Gamma_0. How can I give a
"predicative" proof of induction up to that ordinal? What breaks
down at Gamma_0?

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