Equivalence of the Fermat potential and the lensing potential approaches
  to computing the integrated Sachs-Wolfe effect
  We show in detail that the recently derived expression for evaluating the
integrated Sachs-Wolfe (ISW) temperature shift in the cosmic microwave
background (CMB) caused by individual embedded (compensated) lenses is
equivalent to the conventional approach for flat background cosmologies. The
conventional approach requires evaluating an integral of the time derivative of
the lensing potential, whereas the new Fermat potential approach is simpler and
only requires taking a derivative of the potential part of the time delay.
