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I wrote a module that computes the 4-th order finite difference derivative of a function at $N$ equidistant points of an interval and the difference between the exact and numerical value at those points, and tested it on the function $f(x)=e^{-x^2}$. I would then like to show that the mean error $<e>= \frac{1}{N} \sum_{i=0}^{N} |e_i|$ (where $e_i$ is the difference at the $i$-th point of the interval) decreases proportionally with $N^{-4}$, by experimenting with different values for $N$. I chose $N=\{2^4,2^5,...,2^{15}\}$, but the computation is very slow and the listplot does not even show that the mean error decreases proportionally with $N^{-4}$, why does all that happen? Thanks in advance!

der4[gridpoints_,values_,order_]:= Module[{x=gridpoints, y=values, k=order}, 
de=NDSolve`FiniteDifferenceDerivative[Derivative[k],N[x],"DifferenceOrder"->4]@"DifferentiationMatrix"//Normal; de.N[y]]
number=31;
f[x_]:=Exp[-8*x^2];
grid=Subdivide[-1, 1, number];
val=f[grid];
approx=der4[grid,val,1]
difference=approx-f'[grid];
ListPlot[difference, PlotLabel->"Difference between numerical and analytical solution for f(x)", AxesLabel->{x, "Difference"}]

Num=ConstantArray[0.,12];
Do[Num[[n]]=2^(n+3), {n, 1, 12}];
err=ConstantArray[0.,12];
Do[grid1=Subdivide[-1, 1, Num[[n]]]; err[[n]]=(1/Num[[n]])*Total[Abs[der4[grid1,f[grid1],1]-f'[grid1]]], {n, 1, 12}];
ListPlot[Transpose[{Num^(-4), err}], AxesLabel->{"N^(-4)","Mean error"}, PlotLabel->"Mean error decrease"]
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    – bbgodfrey
    Nov 20, 2021 at 20:43
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    $\begingroup$ To speed it up, get rid of the Normal. The SparseArray representation of your very large differentiation matrix is your friend. Otherwise for n = 11, 12 you spend most of the time swapping (my memory usage said 15+GB). If I use ListLogLogPlot, I see the decrease, up to a point that's probably expected. $\endgroup$
    – Michael E2
    Nov 20, 2021 at 21:40
  • $\begingroup$ @MichaelE2 Thank you! $\endgroup$
    – JBuck
    Nov 20, 2021 at 23:11

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