Any plan for including multidimensional linear operators?

[ehgus]

Hello, do you have a plan to include array-to-arrat feature in LinearOperator?
The current LinearOperator only support vector-to-vector.
This is not enough because I need to do redundant reshaping before applying multidimensional linear functions such as fft.

I hope it can explicitly support such linear functions.

[dpo]

Do you mean something like

op = LinearOperator(…)
A = rand(m, n)
B = op * A

If so, you should already be able to do it, but B is another linear operator. You can materialize it with Matrix(B).

[ehgus]

What I mean is like

sz = (6,6,6)
op = LinearOperator(Float64, sz, sz, ...) # input: array with size (6,6,6) -> output: array with size (6,6,6) 
A = rand(sz)
B = op * A # return array with size (6,6,6) 

There are list of linear functions for tensor available in Julia.

I have used a trick to use such functions. For example, when doing 3-dim FFT (input: 3-dim array, output: 3-dim array), I give a vectorized array to LinearOperator and then convert it to the original shape before FFT.

using LinearOperators
using FFTW

sz = (6,6,6)
X = randn(ComplexF64, sz) |> vec;

function reshape_fft!(res, v, α, β)
    v = reshape(v,sz)
    if β == 0
        res .= α .* vec(fft(v))
    else
        res .= α .* vec(fft(v)) .+ β .* res
    end
end

function reshape_ifft!(res, v, α, β)
    v = reshape(v,sz)
    if β == 0
        res .= α .* vec(ifft(v))
    else
        res .= α .* vec(ifft(v)) .+ β .* res
    end
end

dft = LinearOperator(ComplexF64, prod(sz), prod(sz), false,false, reshape_fft!, nothing, reshape_ifft!)

rst =  reshape(dft * X, sz)