  By means of small magnetic field ac susceptibility measurement at 10 kHz we
found that the real and imaginary parts of ceramic $YBa_2Cu_3O_ x$
susceptibility in presence of the external low frequency field close to 0.1 Hz
exhibit frequency dependence. The wide maximum of hysteresis losses and
{\color{black} the exponential dependence} of the {\color{black} effectiveness
of flux penetration} with increasing of external field frequency were obtained.
We observed a nonlinear dynamic magnetic response in presence dc field and we
suggest that this behavior is due to the dynamic and static Josephson
vortex-vortex interaction.
