# Dense networks that do not synchronize and sparse ones that do

## TL;DR

This paper investigates the critical connectivity thresholds for synchronization in Kuramoto oscillator networks, providing new bounds for dense networks and partial results for sparse networks, highlighting unresolved questions in network synchronization.

## Contribution

It offers a refined lower bound on the critical connectivity for global synchronization and explores how minimal edge additions can induce synchronization in sparse networks.

## Key findings

- Critical connectivity may be exactly 75% for circulant networks.
- Improved lower bound on critical connectivity from 68.18% to 68.28%.
- Adding O(n log n) edges can destabilize twisted states in ring networks of size 2^m.

## Abstract

For any network of identical Kuramoto oscillators with identical positive coupling, there is a critical connectivity above which the system is guaranteed to converge to the in-phase synchronous state, for almost all initial conditions. But the precise value of this critical connectivity remains unknown. In 2018, Ling, Xu, and Bandeira proved that if each oscillator is coupled to at least 79.29 percent of all the others, global synchrony is ensured. In 2019, Lu and Steinerberger improved this bound to 78.89 percent. Here, by focusing on circulant networks, we find clues that the critical connectivity may be exactly 75 percent. Our methods yield a slight improvement on the best known lower bound on the critical connectivity, from $68.18\%$ to $68.28\%$. We also consider the opposite end of the connectivity spectrum, where the networks are sparse rather than dense. In this regime, we ask how few edges one needs to add to a ring of $n$ oscillators to turn it into a globally synchronizing network. We prove a partial result: all the twisted states in a ring of size $n=2^m$ can be destabilized by adding just $\mathcal{O}(n \log_2 n)$ edges. To finish the proof, one also needs to rule out all other candidate attractors. We have done this for $n=8$ with computational algebraic geometry, but the problem remains open for larger $n$. Thus, even for systems as simple as Kuramoto oscillators, much remains to be learned about dense networks that do not globally synchronize and sparse ones that do.