# Truncated basis ApproxFun

**URL:** https://discourse.julialang.org/t/truncated-basis-approxfun/39495
**Category:** General Usage
**Tags:** fftw
**Created:** [May 14, 2020, 10:57pm UTC](https://discourse.julialang.org/t/truncated-basis-approxfun/39495 "2020-05-14T22:57:13Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![mleprovost](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mleprovost/32/7166_2.png) [@mleprovost](https://discourse.julialang.org/u/mleprovost)
#### Post date: [May 14, 2020, 10:57pm UTC](https://discourse.julialang.org/t/truncated-basis-approxfun/39495/1 "2020-05-14T22:57:13Z")

</div>

Hello,

I would like to use ApproxFun.jl for a truncated FFT. Is it possible to create direct and inverse Fourier transforms spanned by a sparse basis?

This is what I have in mind:

```julia
using ApproxFun

S = Fourier()

n = 256
sparse_idx = [1; 20; 30; 40]
T = ApproxFun.plan_transform(S, n)
Ti = ApproxFun.plan_itransform(S, n)

T_sparse = ApproxFun.plan_transform(S, n, sparse_idx)
Ti_sparse = ApproxFun.plan_itransform(S, n, sparse_idx)

```