Efficient polynomial evaluation using idea of fast fourier transform

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Please would anyone suggest an efficient algorithm ($O(n \log n)$) to evaluate a polynomial at all the $n$th roots of unity, where $n$ is not a power of $2$?

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Look up Prime-factor FFT algorithm, Bruun's FFT algorithm, Rader's FFT algorithm, Bluestein's FFT algorithm, and Vinograd FFT. Reading the wikipedia article on FFT should have given you these methods already under the "other FFT methods" section.

FFTW and the Spiral project have pointers on how to construct the chain of methods. Which they also implement.