  One of the models of intermittency is on-off intermittency, arising due to
time-dependent forcing of a bifurcation parameter through a bifurcation point.
For on-off intermittency the power spectral density of the time-dependent
deviation from the invariant subspace in a low frequency region exhibits
1/\sqrt{f} power-law noise. Here we investigate a mechanism of intermittency,
similar to the on-off intermittency, occurring in nonlinear dynamical systems
with invariant subspace. In contrast to the on-off intermittency, we consider
the case where the transverse Lyapunov exponent is zero. We show that for such
nonlinear dynamical systems the power spectral density of the deviation from
the invariant subspace can have 1/f^\beta form in a wide range of frequencies.
That is, such nonlinear systems exhibit 1/f noise. The connection with the
stochastic differential equations generating 1/f^\beta noise is established and
analyzed, as well.
