Investigating ultra-long gravitational waves with measurements of
  pulsars rotational parameters
  A method is suggested to explore the gravitational wave background (GWB) in
the frequency range from $10^{-12}$ to \hbox{$10^{-8}$ Hz}. That method is
based on the precise measurements of pulsars' rotational parameters: the
influence of the gravitational waves (GW) in the range will affect them and
therefore some conclusions about energy density of the GWB can be made using
analysis of the derivatives of pulsars' rotational frequency. The calculated
values of the second derivative from a number of pulsars limit the density of
GWB $\Omega_{gw}$ as follows: $\Omega_{gw}<2\times10^{-6}$. Also, the time
series of the frequency $\nu$ of different pulsars in pulsar array can be
cross-correlated pairwise in the same manner as in anomalous residuals analysis
thus providing the possibility of GWB detection in ultra-low frequency range.
