  A new, computationally- and statistically-efficient algorithm, the Fast
$\chi^2$ algorithm, can find a periodic signal with harmonic content in
irregularly-sampled data with non-uniform errors. The algorithm calculates the
minimized $\chi^2$ as a function of frequency at the desired number of
harmonics, using Fast Fourier Transforms to provide $O (N \log N)$ performance.
The code for a reference implementation is provided.
