  Whenever we have a group acting on a class of functions by translation, the
bispectrum offers a principled and lossless way of representing such functions
invariant to the action. Unfortunately, computing the bispectrum is often
costly and complicated. In this paper we propose a unitarily equivalent, but
easier to compute set of invariants, which we call the skew spectrum. For
functions on homogeneous spaces the skew spectrum can be efficiently computed
using some ideas from Clausen-type fast Fourier transforms.
