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- From: weemba@sagi.wistar.upenn.edu (Matthew P Wiener)
- Newsgroups: sci.math
- Subject: Re: Axioms of set theory, infinity and R. Rucker
- Message-ID: <96652@netnews.upenn.edu>
- Date: 8 Nov 92 15:43:45 GMT
- References: <1992Nov6.133138.16642@prl.philips.nl>
- Sender: news@netnews.upenn.edu
- Reply-To: weemba@sagi.wistar.upenn.edu (Matthew P Wiener)
- Organization: The Wistar Institute of Anatomy and Biology
- Lines: 46
- Nntp-Posting-Host: sagi.wistar.upenn.edu
- In-reply-to: schiller@prl.philips.nl (schiller c)
-
- In article <1992Nov6.133138.16642@prl.philips.nl>, schiller@prl (schiller c) writes:
- >Reading the book "infinity and the mind" by Rudy Rucker
- >(by the way, it is delighting),
-
- Based on his science fiction, which I have found unreadable in large
- doses (too much gee-whiz), I had low expections for IATM. I'm pleased
- to say that I was completely wrong. It's an excellent book.
-
- >Which of these is the infinity specified in the
- >axioms of set theory ?
-
- The basic ZFC axioms specify only the smallest infinity, aleph_0.
- From this, one derives within ZFC aleph_1, aleph_2, ....
-
- > Is it important to decide this
- >question ? Does this have any effect on set theory ?
-
- However, not only are there higher infinities, there are also what may
- be called "stronger" infinities. These can be thought of as being
- unreachable from below under axiomatic expansion. For example, aleph_0
- can not be reached by finite operations done on finite sets. Similarly,
- the stronger cardinals mentioned in IATM cannot be reached from below,
- even using infinitary ZFC operations on infinite, yet smaller, sets.
-
- What's especially intriguing is that these stronger infinities form,
- more or less, a hierarchy of strength. Near the bottom is something
- called an inaccessible cardinal. Near the middle is something called
- a measurable cardinal. Near the top is something called a supercompact
- cardinal. Letting I, M, S denote the existence axiom for the respective
- cardinals, we can form set theories ZFC+I, ZFC+M, ZFC+S. Then we have
- that ZFC+I is stronger than ZFC, in the sense that it is impossible
- to prove there are any inaccessible cardinals just using ZFC, and the
- same for ZFC+M versus ZFC+I, and for ZFC+S versus ZFC+M. Yet the reverse
- holds in each case. There is definitely more mathematics that can be
- proven in the stronger axiom systems.
-
- In fact, this can be made very concrete. There are, thanks to the work
- of Matiyasevich and Jones, all but explicit diophantine equations that
- can be proven to have no solution only in a stronger set theory. It is
- remotely conceivable that Fermat's last theorem even is an example of
- such a diophantine equation.
-
- (Here's a boo in the the general direction of M&J: why not give the
- simpler exponential diophantine equations also?)
- --
- -Matthew P Wiener (weemba@sagi.wistar.upenn.edu)
-