  We study the motion of tiny heavy inertial particles advected by a two
dimensional inviscid fluid flow composed of $N$ identical point vortices
regularly placed on a ring, and forming a crystal. In the limit of weak
particle inertia, we show asymptotically that, in the reference frame of the
crystal, inertial particles have $N$ asymptotically stable equilibrium
positions located outside the crystal, in agreement with numerical observations
by Ravichandran et al. (Sadhana 42, 2017). In addition to these "satellite"
attracting points, we observe that for $N \ge 3$ the center of the ring, though
degenerate, is a stable equilibrium position for inertial particles. This
creates a kind of cage effect, where inclusions slowly drift towards the center
under the effect of the surrounding vortices. This cage effect is observed to
persist even at larger Stokes numbers, in contrast with the satellite
attracting points that vanish when the Stokes number is above some critical
value.
