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Path: utzoo!watmath!watdaisy!ndiamond
From: ndiamond@watdaisy.UUCP (Norman Diamond)
Newsgroups: net.puzzle
Subject: Re: Loop termination proof problem.
Message-ID: <7085@watdaisy.UUCP>
Date: Mon, 18-Mar-85 14:24:12 EST
Article-I.D.: watdaisy.7085
Posted: Mon Mar 18 14:24:12 1985
Date-Received: Tue, 19-Mar-85 03:45:57 EST
References: <1283@amdahl.UUCP>
Distribution: net
Organization: U of Waterloo, Ontario
Lines: 24

>    do until i=1                      /* Start loop                     */
>       if i/2 = trunc(i/2)            /* Q. Is "i" even?                */
>        then i = i/2                  /* A. Yup. Divide by 2            */
>        else i = 3*i + 1              /* A. No. Multiply by 3 and add 1 */
>       say i                          /* Type it for user               */
>    end                               /* End of loop                    */
> 
> The challenge  is to  prove that  this program  will terminate  for all
> positive integer values of "i".  I suspect that it's unprovable...
> -- Kris Stephens

It's been an open problem in pure mathematics for several years.  A similar
problem with around 17 variables was proven undecidable.  I don't follow
such things though.

-- 

   Norman Diamond

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