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MathGroup Archive 1999

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Precision Problems

  • To: mathgroup at
  • Subject: [mg19210] Precision Problems
  • From: "Holger Strauss" <strauss at>
  • Date: Tue, 10 Aug 1999 02:53:00 -0400
  • Organization: Ruhr-Universitaet Bochum, Rechenzentrum
  • Sender: owner-wri-mathgroup at


I have some numerical precision problems (Mathematica 3.0)
with the following code:

f3[x_] := 20 Log[10, 1/(15120 x^4)*(-90720 - 15120 x^2 + 13424 x^4 +
              9 (13440 + 2240  x^2 + 257 x^4) Cos[x] -
              72 (420 + 70  x^2 + 9 x^4) Cos[2x] +
              31 x^4 Cos[3x])]
LogLinearPlot[f3[x], {x,0.05,Pi}]

As can be seen from the plot, evaluating f3 doesn't work for
small arguments. It seems to me that this is the result of large
numbers being subtracted from each other resulting in a very
small difference.
It it possible to increase the precision of the calculation adequatly?
I already tried to encapsulate the whole function with N[..., 50000],
however, without success...
Is there any other way of successfully evaluating and plotting
f3 for small arguments?


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