Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!gegeweb.42!gegeweb.eu!nntpfeed.proxad.net!proxad.net!feeder1-1.proxad.net!198.186.194.247.MISMATCH!transit3.readnews.com!news-out.readnews.com!news-xxxfer.readnews.com!panix!not-for-mail From: JohnF Newsgroups: sci.logic,rec.puzzles,sci.math.num-analysis,comp.programming Subject: Re: the hourglass problem Date: Fri, 6 Jan 2012 06:42:22 +0000 (UTC) Organization: PANIX Public Access Internet and UNIX, NYC Lines: 30 Message-ID: References: <7f48dfe5-ca54-49ca-8cb9-5ae686e96f92@c42g2000prb.googlegroups.com> <4F05AF0A.6BB2B4C3@btinternet.com> NNTP-Posting-Host: panix3.panix.com X-Trace: reader1.panix.com 1325832142 26302 166.84.1.3 (6 Jan 2012 06:42:22 GMT) X-Complaints-To: abuse@panix.com NNTP-Posting-Date: Fri, 6 Jan 2012 06:42:22 +0000 (UTC) User-Agent: tin/2.0.0-20110823 ("Ardenistiel") (UNIX) (NetBSD/5.1 (i386)) Xref: x330-a1.tempe.blueboxinc.net comp.programming:1230 In sci.logic Patricia Shanahan wrote: > Frederick Williams wrote: >> RichD wrote: >>> You're given a 7 minute, and a 4 minute, >>> sand hourglass. >> >> What may one do with these hourglasses? >> Turn them over, certainly, but may one place >> them on their side so that they stop "ticking"? > > Could someone with an actual hourglass check whether that works? What's the diff? As per my preceding post, the more interesting abstract problem is: given a particular "instruction set" (that's apparently not Turing complete), what's the "computable" subset of N? Here, the available hourglasses are maybe like registers or memory locations (I'm not sure what a good computer analogy would be here), and the turn-over operations are like the instruction set. The contents of these "memory locations" aren't really bits, or even bit-like, and so the instructions don't do the usual things, and therefore presumably don't have the usual semantics (though it seems pretty easy to associate them with subsets of N). What seems interesting to me is to try to firm up this kind of analogy, and see if anything worth talking about emerges. -- John Forkosh ( mailto: j@f.com where j=john and f=forkosh )