Kolmogorov stated,
and Zahorski proved, that there exists an L2- Fourier series such that some
rearrangement of it diverges almost everywhere. Kac and Zygmund asked if
the set of rearrangements which make this Fourier series diverge almost
everywhere is first category or second category. A general theorem is proved
which has as a special case that the set of rearrangements in question is
residual.