Linear stability of elliptic Lagrangian solutions of the planar three-body problem via index theory

It is well known that the linear stability of Lagrangian elliptic equilateral triangle homographic solutions in the classical planar three-body problem depends on the mass parameter $\bb=27(m_1m_2+m_2m_3+m_3m_1)/(m_1+m_2+m_3)^2\in [0,9]$andtheeccentricity$e\in [0,1)$. We are not aware of any existing analytical method which relates the linear stability of these solutions to the two parameters directly in the full rectangle $[0,9]\times [0,1)$, besides perturbation methods for $e>0$ small enough, blow-up techniques for $e$ sufficiently close to 1, and numerical studies. In this paper, we introduce a new rigorous analytical method to study the linear stability of these solutions in terms of the two parameters in the full $(\text{bb},e)$ range $[0,9]\times [0,1)$ via the $\text{om}$-index theory of symplectic paths for $\text{om}$ belonging to the unit circle of the complex plane, and the theory of linear operators. After establishing the $\text{om}$-index decreasing property of the solutions in $\text{bb}$ for fixed $e\in [0,1)$, we prove the existence of three curves located from left to right in the rectangle $[0,9]\times [0,1)$, among which two are -1 degeneracy curves and the third one is the right envelop curve of the $\text{om}$-degeneracy curves for $\text{om}≠1$, and show that the linear stability pattern of such elliptic Lagrangian solutions changes if and only if the parameter $(\text{bb},e)$ passes through each of these three curves. Interesting symmetries of these curves are also observed. The singular case when the eccentricity $e$ approaches to 1 is also analyzed in details concerning the linear stability.