“…The ring problem studies the motion of (n + 1)-bodies where n bodies of equal masses are located at the vertices of a regular polygon centered at the remaining body, thus forming a central configuration. It was proposed by Maxwell in [17] as a model for the motion of the particles surrounding Saturn, and used more recently to model systems like planetary rings, asteroid belts, planets around a star, certain stellar formations, stars with accretion ring, planetary nebula, motion of an artificial satellite about a ring, (see [23,25,24,19,20,12,13,3,9,4,5]). We remark that the ring problem with four equal masses on the ring and a fifth mass at the center of the ring considered in [23] coincides with the special case found in Theorem 1.1 (a); such a configuration has been found to be locally unstable.…”