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Interpolation and plot doing strange things with mathematica 6 tia

  • To: mathgroup at smc.vnet.net
  • Subject: [mg88964] Interpolation and plot doing strange things with mathematica 6 tia
  • From: ratullochjk2 at gmail.com
  • Date: Thu, 22 May 2008 02:38:27 -0400 (EDT)

Greetings All

I run these commands that I've had plenty of help getting to
work...but the plot doesn't follow the points, as you can see in the
plot.  At the end it just goes crazy and when I test the points with

input s[0]
output is -3.73293
why are the numbers so out of sync... at s[0] it should be -2 not
-3.73293

I try increasing the number in the line
N[FourierTrigSeries[f[x], x, 25, FourierParameters -> {-1, 1/138}]]
(I've changed the number 25 to over 100 and the problem seems to get
worse)

See code below:

In[89]:= data3 = {{0, -2}, {7, 1}, {10, 6}, {12, -2}, {18, -6}, {27,
6}, {34,
    4}, {42, -6}, {49, -5}, {56, 4}, {62, 3}, {67, -5}, {70, -3}, {79,
     3}, {83, 7}, {88, -3}, {89, -8}, {96, 7}, {105,
    9}, {113, -8}, {122, -9}, {131, 9}, {132,
    1}, {134, -9}, {138, -2}};
f = Interpolation[data3, PeriodicInterpolation -> True];

<< "FourierSeries`"

s[x_] = N[
  FourierTrigSeries[f[x], x, 25, FourierParameters -> {-1, 1/138}]]

discr = Interpolation[data3 /. {x_, y_} -> {x, y},
   InterpolationOrder -> 1];

g[x_] = Piecewise[{{discr[x], 0 < x < 138}, {0, True}}];

Show[Plot[s[x], {x, 0, 138}, PlotStyle -> Red,
  PlotRange -> {-15, 15}], Plot[g[x], {x, 0, 138}, Filling -> Axis],
 ListPlot[data3, Filling -> Axis, PlotRange -> {0, 138}]]

tia sal2


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