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problems with ContourPlot


Hi out there,


if I try this:


a := 0.5; b := 0.5; c := 0.5; y := 0.0;

ContourPlot[(x*(x - a) + y*y + z*z)/ (Sqrt[(x*x + y*y + z*z) ** 3]*
          Sqrt[((x - a) ** 2 + y*y + z*z) ** 3]) - ((x - a)*(x - a - b
-c) +
          y*y + z*z)/ (Sqrt[((x - a) ** 2 + y*y + z*z) ** 3]*
          Sqrt[((x - a - b - c) ** 2 + y*y + z*z) ** 3]) - (x*(x - a -b)
+
          y*y + z*z)/ (Sqrt[(x*x + y*y + z*z) ** 3]*
          Sqrt[((x - a - b) ** 2 + y*y + z*z) **
              3]) + ((x - a - b)*(x - a - b - c) + y*y +
          z*z)/(Sqrt[((x - a - b) ** 2 + y*y + z*z) ** 3]*
          Sqrt[((x - a - b - c) ** 2 + y*y + z*z) ** 3]), {x, -1, 1},
{z, 0, 1}, Contours -> {0, 1, 5, 10, 50, 100}, ContourShading -> False]




I receive the error-messages:



[ ...Expression... ] is not a machine-size real number at {x, z} =
{-1.,0.}


ContourGraphics::"ctpnt": "The contour is attempting to traverse a cell
in which some of the points have not evaluated to numbers, and it will
be dropped."


General::"stop": "Further output of (ContourGraphics :: "ctpnt") will be
suppressed during this calculation."





Because I compiled this also with Fortran I know that there are also
results in the range up to 10^6. Is this too much for ContourPlot? Or
did I some crazy things.... ?? At (-1,0.) I  cannot find any
singularity...



Thank you very much for any small hint.

Harry







--
Harald von der Osten-Woldenburg
Geophysical Prospection of Archaeological Sites
National Heritage Department of Baden-Wuerttemberg
Silberburgstrasse 193, D-70178 Stuttgart
Fax Office: +49-(0)711-1694-707
http://www.lb.netic.de/hvdosten : Geomagnetics, Geoelectrics, Radar, EMI






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