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Re: Numeric warnings during symbolic manipulation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112779] Re: Numeric warnings during symbolic manipulation
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Thu, 30 Sep 2010 04:51:31 -0400 (EDT)
  • References: <201009290815.EAA28182@smc.vnet.net>

Yaroslav Bulatov wrote:
> Last two expressions are equivalent. Why does the second one gives
> "N::meprec" warnings? Also, is there a good way to stop Mathematica
> from running N on my intermediate expressions?
> 
> a = 2/(4 + 1/E^2 + 2 E^2 + E^6) + E^2/(4 + 1/E^2 + 2 E^2 + E^6) +
>    E^6/(4 + 1/E^2 + 2 E^2 + E^6);
> b = 4/(4 + 1/E^2 + 2 E^2 + E^6) + 1/(
>    E^2 (4 + 1/E^2 + 2 E^2 + E^6)) + (2 E^2)/(
>    4 + 1/E^2 + 2 E^2 + E^6) + E^6/(4 + 1/E^2 + 2 E^2 + E^6);
> Exp[Log[a/b]]
> Exp[Log[a] - Log[b]]

There is no compelling reason for that message. There is a reason for 
using numerical evaluation. In this case it is to assess imaginary part 
of the exponent, in order to reduce by an appropriate integral multiple 
of Pi. Regardless of whether one regards this as good or bad, I am not 
aware of any way to prevent it.

Daniel Lichtblau
Wolfram Research


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