Almost-global convergence to twisted states in discrete Huygens rings

Abstract

We study the discrete Huygens phase map on the ring $C_n$, the simplest oscillator network in which the coupling can be frustrated. Reciprocal coupling favours phase opposition between neighbours, while the edge phase differences must sum to zero modulo $2π$ around the cycle; for odd $n$ these two requirements are incompatible. The parity of $n$ determines the absolute minima of the potential. When $4\mid n$, the critical set also contains circles of critical points, all of which meet at two completely degenerate configurations. After quotienting by the common phase, the map is a gradient step for an intrinsic potential on an $(n-1)$-torus, and the cycle spectrum gives an explicit interval of interaction strengths on which every orbit converges to a critical point. Our main result is that, on this whole interval and for every $n\ge3$, almost every orbit converges to a uniform twisted minimum, including at interaction strengths for which the map is not invertible. When $4\mid n$, the non-minimal points of the critical circles are treated by a centre-stable manifold argument. At the two degenerate intersections, a local blow-up reduces the problem to the cubic leading term of the potential and its restriction to the unit sphere. Orbits approaching along directions where this restriction is positive form a null set, because the cubic is harmonic in the intrinsic metric. No orbit approaches along its zero level, where the critical points are non-degenerate saddles on invariant rays. Hence the degenerate equilibria also have basins of zero measure.