For an integer N, let be a not unit N'th root of unity.
We consider the following discrete Fourier transform given by the symmetric Vandermonde matrix:
The square of the Fourier transform is the identity transform:
Exercise (*). If a network w/conductivity is rotation invariant then its Dirichlet-to-Neumann operator is diagonal in the Fourier coordinates. (Hint) The -harmonic functions commute w/rotations.