From: "John Baez" <baez@math.ucr.edu>
To: categories@mta.ca (categories)
Subject: Re: fundamental theorem of algebra
Date: Mon, 3 Apr 2006 16:41:24 -0700 (PDT) [thread overview]
Message-ID: <E1FQZWY-0001F2-9h@mailserv.mta.ca> (raw)
Vaughan writes:
> However I've been reflecting on just what is behind the very uniform
> insistence on the distinction between an algebraic proof and an analytic
> one. Since algebra is descended from analysis, it seems unkind for
> algebra to deny its parentage in this way.
I thought people knew how to add before they knew how to take limits. :-)
> But I see now that this denial is logically necessary. For consider the
> algebraic plane, the least algebraically closed subfield of the complex
> plane, consisting of the algebraic numbers. The FTAlg is by definition
> true there, so it ought to be provable there.
Hmm. How do you propose to show there *exists* an algebraically closed
subfield of the complex numbers? I would do it using the fundamental
theorem of algebra - the usual one, for the complex numbers. Unless
you have some other way, I don't understand how you hope to circumvent
the use of analysis by introducing such an entity.
Indeed, the usual proof that the real numbers contains a square root
of 2 uses the completeness of the real numbers, which also counts as
"analysis".
> It is ironic that a theorem of algebra about an algebraic domain that
> itself has no element of analysis to it, being just the algebraic
> closure of the rationals, a small and totally disconnected space, should
> require analysis, the parent of algebra, for its proof.
That the rational numbers has an algebraic closure is a purely algebraic
result, with no mention of topology in either the statement or proof.
That the complex numbers is algebraically closed is not an algebraic result:
it has topology built into the statement, and also the proof(s).
That the algebraic closure of the rationals embeds in the complex
numbers has topology in the statement - and I bet also in every proof.
Best,
jb
next reply other threads:[~2006-04-03 23:41 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-03 23:41 John Baez [this message]
-- strict thread matches above, loose matches on Subject: below --
2006-04-04 1:08 Michael Barr
2006-04-03 4:18 Vaughan Pratt
2006-04-02 18:43 Fred E.J. Linton
2006-04-02 0:59 Vaughan Pratt
2006-04-01 14:59 jim stasheff
2006-04-01 13:01 Michael Barr
2006-04-01 9:44 Prof. Peter Johnstone
2006-03-31 19:39 John Baez
2006-03-31 7:20 Vaughan Pratt
2006-03-31 4:01 John Baez
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