Re: FullSimplify Hang?

*To*: mathgroup at smc.vnet.net*Subject*: [mg62228] Re: FullSimplify Hang?*From*: carlos at colorado.edu*Date*: Wed, 16 Nov 2005 02:28:29 -0500 (EST)*References*: <200511140216.VAA23580@smc.vnet.net><dlc9hn$m9o$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

FullSimplify does not report progress. There is a time constraint option (also in Simplify), but it applies only to individual transformations, not the entire process. Killing the process leaves the original expression unchanged, so it is all or nothing. If the original expression is too complicated to finish in reasonable time (say a weekend) human intervention is necessary. What kind of intervention? Typically divide and conquer. Some examples: Extract numerator and denominator from fractions, fullsimplify each and recombine. Break up complicated expressions into groups (that has to be done by hand, using InputForm and cut&paste) fullsimplify each group, recombine. Hard when many fraction levels appear. For lists and matrices simplify an item at a time in a loop, and observe which items cause fullsimplify to hang. Separate those for further divide and conquer. Changing target functions can be effective for complex expressions. Changing complexity measure sometimes helps, but effect is random. Do simplify first, then fullsimplify, kill latter if it hangs. All of these gyriations could be avoided by putting a time constraint on the full process, and asking fullsimplify to return the simplest one found so far, with a warning.

**References**:**FullSimplify Hang?***From:*"Igor Touzov" <igor@nc.rr.com>