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MathGroup Archive 2006

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Re: Plot3D: plot does not display when PlotPoints different from 25. Mathematica 5.2

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65794] Re: Plot3D: plot does not display when PlotPoints different from 25. Mathematica 5.2
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Mon, 17 Apr 2006 02:28:50 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

On 4/16/06 at 3:49 AM, nma at 12000.org wrote:


>Remove["Global`*"]

>u[x_, y_] := Module[{},
>If[x == 0, Return[1 + y]]; If[y == 0, Return[1]]; Return[0];
>]

>Plot3D[u[x, y], {x, -1, 2}, {y, -1, 2},
>FaceGrids -> None, AxesLabel -> {"U(x,y)", "x", "y"}, PlotLabel ->
>"Boundary conditions", PlotPoints -> 25]

>works OK. But if you change 25 to say 26 in the last line above, the
>plot does not show up any more !

>Can someone explain why this is?

Your function is everywhere 0 except for points on either the x or y axis. So, unless Mathematica happens to sample at either the x or y axis, it sees the function as constant. Changing the plot points changes where Mathematica does an initial sample. And obviously when the setting PlotPoints->26, the initial sample doesn't include a point on either the x or y axis.
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