  Consider a rigid body, $\mathscr B$, constrained to move by translational
motion in an unbounded viscous liquid. The driving mechanism is a given
distribution of time-periodic velocity field, $\bfv_*$, at the interface
body-liquid, of magnitude $\delta$ (in appropriate function class). The main
objective is to find conditions on $\bfv_*$ ensuring that $\mathscr B$ performs
a non-zero net motion, namely, $\mathscr B$ can cover any given distance in a
finite time. The approach to the problem depends on whether the averaged value
of $\bfv_*$ over a period of time is (case (b)) or is not (case (a))
identically zero. In case (a) we solve the problem in a relatively
straightforward way, by showing that, for small $\delta$, it reduces to the
study of a suitable amd well-investigated time-dependent Stokes (linear)
problem. In case (b), however, the question is much more complicated, because
we show that it {\em cannot} be brought to the study of a linear problem.
Therefore, in case (b), self-propulsion is a genuinely nonlinear issue that we
solve directly on the nonlinear system by a contradiction argument approach. In
this way, we are able to give, also in case (b), sufficient conditions for
self-propulsion (for small $\delta$). Finally, we demonstrate, by means of
counterexamples, that such conditions are, in general, also necessary.
