A Fourier-like transform is defined over a ring of quadratic integers modulo a prime number

in the quadratic field

, where

is a square-free integer. If

is a Fermat prime, one can utilize the fast Fourier transform (FFT) algorithm over the resulting finite fields to yield fast convolutions of quadratic integer sequences in

. The theory is also extended to a direct sum of such finite fields. From these results, it is shown that Fourier-like transforms can also be defined over the quadratic integers in

modulo a nonprime Fermat number.