  We derive for a non-relativistic system an approximation for Final State
Interactions in a form, resembling a DWIA which corrects the structure function
computed in the PWIA. We then compare the Gersch-Rodriguez-Smith and the IA
series for structure functions and prove that to order ${\cal O}(1/q^2)$ the
above DWIA representation is contained in the GRS series to the same order.
There is an additional term in the GRS series that is missing in the DWIA due
to the eikonal approximations in the latter. This strongly suggests that the
two approaches, when treated exactly, produce identical structure function to
arbitrary order in $1/q$.
