  The net steady state flow pattern of a distorting sphere is studied in the
framework of the bilinear theory of swimming at low Reynolds number. It is
argued that the starting point of a theory of interacting active particles
should be based on such a calculation, since any arbitrarily chosen steady
state flow pattern is not necessarily the result of a swimming motion.
Furthermore, it is stressed that as a rule the phase of stroke is relevant in
hydrodynamic interactions, so that the net flow pattern must be used with
caution.
