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I need to generate a table of Chebyshev expansion coefficients of trigonometric functions (in this case Cos[2 Pi t] to very high accuracy. Code is:

ClearAll["Global`*"]

m = 4;

f[t] = Cos[2 Pi t];
Tn[t] = ChebyshevT[j, 2*t - 1];
wt[t] = 1/Sqrt[t - t^2];    

p = Table[
   NIntegrate[f[t]*Tn[t]*wt[t], {t, 0, 1}]/(Pi/2.0), {j, 0, m - 1}];
p[[1]] = p[[1]]/2;

p

Where f[t] is the trigonometric function, Tn[t], is the shifted Chebyshev polynomial, w[t] is the weight function. How can I increase the precision of this numerical integration?

I apologize if this is a duplicate, but I've read dozens of past questions dealing with $MinPrecision, $MinAccuracy, PrecisionGoal, WorkingPrecision, etc. and I've been unable to increase the precision. I'm stuck at machine precision still!

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  • $\begingroup$ you do know dividing by 2.0converts the result to machine precision regardless of what NIntegrate does.. WorkingPrecision -> 20 , PrecisionGoal -> 20 should do what you want if you make that an integer 2 $\endgroup$
    – george2079
    Nov 6, 2014 at 19:30
  • $\begingroup$ @george2079 I did not know that, but now I do. Thank you! $\endgroup$
    – gKirkland
    Nov 6, 2014 at 22:15
  • 1
    $\begingroup$ I'd like to point out that Mathematica can do the calculation symbolically (ie, infinite precision) for $0\leq m\leq 6$ (and possibly higher). So you could just calculate the symbolic results and then apply N. $\endgroup$ Nov 7, 2014 at 1:05

1 Answer 1

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Your results are converted to MachinePrecision because you divide by the machine precision number Pi/2.0. You can get exact solutions with Integrate and use N. This will be more accurate than NIntegrate.

Block[{m = 4},
 p = Table[
    Integrate[f[t]*Tn[t]*wt[t], {t, 0, 1}]/(Pi/2), {j, 0, m - 1}];
 ]
p[[1]] = p[[1]]/2;

p
(*
  {-BesselJ[0, π],
   0,
   2 BesselJ[2, π],
   0}
*)

N[p, 20]
(*
  {0.30424217764409386420,
   0,
   0.97086786526301821941,
   0}
*)
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