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I am trying to plot the partial sums for $N=5,10,15,20$ of the Fourier series of $f(x)=(1-x^2)^2$ and $-1<x<1.$ I have:

f[x_] = If[1 > x > -1, (1-x^2)^2];
L = 2;
a[n_] := (2/L)*Integrate[f[x]*Cos[2 n*Pi*x/L], {x, -L/2, L/2}]
a[0] = (1/L)*Integrate[f[x], {x, -L/2, L/2}]
b[n_] := (2/L)*Integrate[f[x]*Sin[2 n*Pi*x/L], {x, -L/2, L/2}]
F[x_, N_] := 
a[0] + Sum[a[n]*Cos[2 n*Pi*x/L] + b[n]*Sin[2 n*Pi*x/L], {n, 1, N}]
Table[F[x, N], {N, 5, 20, 5}]
p[N_, a_] := 
Plot[Evaluate[F[x, N]], {x, -a, a}, PlotRange -> All, 
PlotPoints -> 200]
a[n]
a[0]
b[n]
Table[p[N, 1], {N, 5, 20, 5}]
g[x_, N_] := Abs[f[x] - F[x, N]]
Table[g[x, 7], {x, -1, 1, 2/10}]

Something is getting wrong. What should I do? Then, how could I use FourierTrigSeries, FourierParameters and FourierCoefficient?

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  • $\begingroup$ what's the definition of a in the plotRange attribute? $\endgroup$
    – Alucard
    Jan 18, 2020 at 15:48
  • $\begingroup$ $a=1$ ($1>x>-1$) $\endgroup$
    – George
    Jan 18, 2020 at 15:50
  • $\begingroup$ ah yes, i didn't realize it. $\endgroup$
    – Alucard
    Jan 18, 2020 at 15:53
  • $\begingroup$ can you help me? $\endgroup$
    – George
    Jan 18, 2020 at 15:54
  • 1
    $\begingroup$ Just a guess. N is a built-in symbol name, try using a different name. $\endgroup$ Jan 18, 2020 at 16:17

1 Answer 1

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ClearAll[g];
ClearAll[uff];
uff[a_, b_] := (FourierSinSeries[(1 - t^2)^2, t, b] + 
    FourierCosSeries[(1 - t^2)^2, t, b] ) /. t -> a
fun = Table[uff[a, i] , {i, 5, 15, 5}];
Plot[fun, {a, -1, 1}]

enter image description here

g[ x_ /; -1 <= x <= 1, n_] := Abs[(1 - x^2)^2 - uff[x, n]]
ListPlot[Table[N@g[x, 5], {x, -1, 1, 2/10}] ]

enter image description here

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