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Re: floating point issue

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86808] Re: [mg86748] floating point issue
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Fri, 21 Mar 2008 01:56:14 -0500 (EST)
  • Reply-to: hanlonr at cox.net

K = Rationalize[{111.5, 10.5, 1.5}, 0];
g = Rationalize[{-0.7071068, 0., -0.7071068}, 0];

K.Cross[K, g]

0

K = {111.5`25, 10.5`25, 1.5`25};
g = {-0.7071068`25, 0.`25, -0.7071068`25};

Chop[K.Cross[K, g], 10^-18]

0


Bob Hanlon

---- Chris Scullard <scullard at uchicago.edu> wrote: 
> Hi everyone,
> 
> I wonder if I can get some opinions on the best way to deal with this 
> precision issue I am having. I define the vectors:
> 
> K = {111.5, 10.5, 1.5}
> g={-0.7071068, 0., -0.7071068}
> 
> And I need this:
> 
> K.Cross[K, g]
> 
> to be 0 in accordance with a vector identity. The answer comes out to 
> around 1.3 x 10^(-13), which is certainly close to 0 but not close 
> enough for what I'm doing. I've tried various things like writing out 
> the cross product explicitly without using the functions but the result 
> is the same. And using N in various places doesn't seem to help either. 
> What's the standard solution for this kind of thing?
> 
> Thanks,
> Chris
> 



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