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Topic: Ultranet
Replies: 2   Last Post: Nov 16, 2012 2:49 AM

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David C. Ullrich

Posts: 21,553
Registered: 12/6/04
Re: Ultranet
Posted: Nov 15, 2012 8:57 AM
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On Thu, 15 Nov 2012 01:28:04 -0800, William Elliot <>

>What's an example of an universal or ultranet?
>Wikipedia claims that if n:D -> X is an ultranet into X and
>f:X -> Y, then the composition f.n:D -> Y is an ultranet into Y.
>I dispute the claim

Did you spend more than a second thinking about it?
The proof is completely and utterly mind-bogglingly

It relies on a few deep observations. Say f : X -> Y
and A is a subset of Y. Say x is an element of x.

(i) Exactly one of the following holds: f(x) is in A,
f(x) is not in A.

(ii) f(x) is not in A if and only if f(x) is in Y \ A.

> but knowing of no examples
>of ultranets, I'm bankrupt for a counter example.

Date Subject Author
Read Ultranet
William Elliot
Read Re: Ultranet
David C. Ullrich
Read Re: Ultranet
William Elliot

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