  The scheme of Clauser and Dowling (Phys. Rev. A 53, 4587 (1996)) for
factoring $N$ by means of an N-slit interference experiment is translated into
an experiment with a single Mach-Zehnder interferometer. With dispersive phase
shifters the ratio of the coherence length to wavelength limits the numbers
that can be factored. A conservative estimate permits $N \approx 10^7$. It is
furthermore shown, that sine and cosine Fourier coefficients of a real periodic
function can be obtained with such an interferometer.
