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From: matt@oddjob.UUCP (Matt Crawford)
Newsgroups: net.physics
Subject: Re: Quickly Computing Quarks (Science News, VOL. 128)
Message-ID: <945@oddjob.UUCP>
Date: Sat, 24-Aug-85 17:39:37 EDT
Article-I.D.: oddjob.945
Posted: Sat Aug 24 17:39:37 1985
Date-Received: Sun, 25-Aug-85 13:27:56 EDT
References: <509@sri-arpa.ARPA> <230@geowhiz.UUCP>
Reply-To: matt@oddjob.UUCP (Matt Crawford)
Organization: U. Chicago, Astronomy & Astrophysics
Lines: 23

In article <230@geowhiz.UUCP> karsh@geowhiz.UUCP (Bruce Karsh) writes:
>Does anybody know how you would go about retaining significant digits
>in a computation like this?  If you figure there there will be about
>10**9 round off errors per second accumulating for one year, there must
>be some plans for designing the calculations to be *EXTREMELY*
>insensitive to round off problems.

I have done calculations of this sort (on a much smaller scale),
and round-off errors do not accumulate.  The system being
simulated is represented by discrete parts and and then each
part is repeatedly altered at random to any of its allowable
states with the probability of each state dependent on the
energy of that state and a parameter which plays the role of
temperature.  As the parameter is gradually reduced, the lowest
energy state of the system should be discovered.

IBM has used this technique to lay out chips on a circuit
board.  The "energy" of a given configuration is a count of
how many wire crossings or how much wire length is needed
to connect the chips.
_____________________________________________________
Matt		University	crawford@anl-mcs.arpa
Crawford	of Chicago	ihnp4!oddjob!matt