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Plot artifact of NDSolve result


Both in Mathematica 8.0.0 and 7.0.1 the plot of NDSolve result is
wrong near x-axis point 60. But when zooming in, plot is correct.

Notebook and screenshot of result are available on http://users.skynet.be/fa991376/

eqn4 = y''[t] == -y[t]^2 + t

sol4 = NDSolve[{eqn4, y[0] == 0, y'[0] == 1}, y, {t, 0, 70},
  WorkingPrecision -> 20, MaxStepSize -> 0.01]

Below plot contains the artifact:

Plot[Evaluate[{y[t], y'[t], y''[t]} /. sol4], {t, 0, 70},
 PlotRange -> All]

Below plot is correct:

Plot[Evaluate[{y[t], y'[t], y''[t]} /. sol4], {t, 50, 70},
 PlotRange -> All]

Is there a way to avoid this?


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