Mass independent shapes for relative equilibria in the two dimensional constant positive curved three body problem

Abstract

In the planar three-body problem under Newtonian potential, it is well known that any masses, located at the vertices of an equilateral triangle generates a relative equilibrium, known as the Lagrange relative equilibrium. In fact, the equilateral triangle is the unique mass independent shape for a relative equilibrium in this problem. The two dimensional positive curved three-body problem, is a natural extension of the Newtonian three-body problem to the sphere $\mathbb{S}^2$, where the masses are moving under the influence of the cotangent potential. S.~Zhu showed that in this problem, equilateral triangle on a rotating meridian can form a relative equilibria for any masses. This was the first report of mass independent shape on $\mathbb{S}^2$ which can form a relative equilibrium. % In this paper, we show that, in addition to the equilateral triangle, there exists one isosceles triangle on a rotating meridian, with two equal angles seen from the centre of $\mathbb{S}^2$ given by $2^{-1}\arccos((\sqrt{2}-1)/2)$, which always form a relative equilibrium for any choice of the masses. Additionally we prove that, the equilateral and the above isosceles relative equilibrium are unique with this characteristic. We also prove that each relative equilibrium generated by a mass independent shape is not isolated from the other relative equilibria.