Static equilibrium of the charged $N$-body problem
We study non-collision static equilibria in the charged \(N\)-body problem. Unlike the classical Newtonian case, such equilibria may exist because the effective interaction coefficients $δ_{ij}=m_im_j-e_ie_j$ can be positive, negative, or zero. We give a complete classification of three-body equilibria and prove that all of them are collinear and linearly unstable. For the four-body problem, we derive sign restrictions for convex and concave non-collinear equilibria, exclude non-trivial concyclic quadrilateral equilibria, and obtain a geometric necessary condition expressed through an auxiliary triangle and a resultant equation. We also study collinear equilibria by reducing the force-balance equations to a homogeneous polynomial system. This yields a resultant necessary condition and an inverse realization theorem showing that every prescribed collinear configuration of distinct points can be realized by suitable positive masses and real charges. Finally, we analyze regular polygon configurations and centered regular polygon configurations, showing in particular that equal masses cannot form a non-trivial regular \(N\)-gon equilibrium, while the centered case reduces to a single scalar condition.