= Solution
For an incompressible Newtonian fluid with negligible inertia and no body force, the <Stokes flow>[Stokes equations] are
$$
\boxed{
-\nabla p+\mu\nabla^2\mathbf u=0,
\qquad
\nabla\cdot\mathbf u=0}.
$$
They are linear because any linear combination of solutions corresponds to the same linear combination of boundary data and applied forces. They are reversible because replacing all imposed velocities and forces by their negatives sends
$$
(\mathbf u,p)\longmapsto(-\mathbf u,-p).
$$
Fluid particles therefore retrace their paths under reversal. This is <kinematic reversibility of Stokes flow>.
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