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- From: rabin@CS.Yale.Edu (Dan Rabin)
- Subject: Re: computable reals
- In-Reply-To: lucas@WATSON.IBM.COM's message of 12 Sep 92 04: 24:12 GMT
- Message-ID: <RABIN.92Sep12155227@epic.CS.Yale.Edu>
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- Organization: Computer Science, Yale University, New Haven, CT 06520-2158
- References: <9209120452.AA03328@ucbvax.Berkeley.EDU>
- Date: Sat, 12 Sep 1992 15:52:27 GMT
- Lines: 43
-
- Bruce Lucas asks about the set of computable reals.
-
- Unfortunately, if you're hoping to use them in programs, numeric
- comparisons are undecidable: Asking whether any arbitrary computable
- real is zero is exactly the same as asking whether its defining
- digit-producing program ever cranks out a non-zero digit. That's not
- decidable. Oops.
-
- Of course, this is not a deficiency with respect to the entire set of
- real numbers, which has this problem plus the problem that some
- (almost all, actually ;-))of its
- elements aren't finitely representable in the first place. It's only
- a deficiency if you thought that moving from floating point to
- computable-reals in a computer program would be a smooth transition.
-
- There is a literature on this subject within computer science.
- Here are a couple of initial pointers:
-
- @incollection{boehm-cartwright:exact-reals,
- author = {Hans Boehm and Robert Cartwright},
- title = {Exact Real Arithmetic: Formulating Real Numbers as
- Functions},
- editor = {David A. Turner},
- chapter = {3},
- booktitle = {Research Topics in Functional Programming},
- year = {1990},
- series = {The UT Year of Programming Series},
- publisher = {Addison-Wesley Publishing Company, Inc.},
- }
-
- @InProceedings{vuillemin:exact-real,
- author = "Jean Vuillemin",
- title = "Exact Real Computer Arithmetic with Continued
- Fractions",
- pages = {14--27},
- booktitle = "Proceedings of the 1988 {ACM} Conference on {L}isp
- and Functional Programming",
- year = "1988",
- month = "July",
- }
-
- -- Dan Rabin
- rabin-dan@cs.yale.edu
-