Detection of extremely low frequency gravitational wave using
  gravitational lens: The general case
  The effect of gravitational wave of extremely low frequency on time delays
between different locations on the Einstein ring in a lens system with an
aligned source-deflector-observer configuration is investigated. The observer
will observe an Einstein ring from the lens system aligned in a highly
symmetric configuration. Time delays between different locations on the
Einstein ring cannot emerge without gravitational wave of cosmological
wavelength propagating through the lens system. Otherwise, different locations
on the Einstein ring will show time delays. Our previous studies showed that
time delays from different locations on the Einstein ring in the presence of
gravitational wave of cosmological wavelength present a special relationship.
But this result is limited to a specific aligned source-deflector-observer
configuration where the source and the observer are equidistant from the
deflector and the gravitational wave has special direction of propagation and
polarization. In order to investigate the property in the general case, we
extend to the general condition that the source and the observer aren't
equidistant from the deflector and the direction and polarization of the
gravitational wave is arbitrary in the present work. Results in this work show
that time delays between different locations on the Einstein ring with the
general condition present the same relationship as that in our previous
studies.
