Flocking dynamics and mean-field limit in the Cucker–Smale-type model with topological interactions

• Flocking behavior of Cucker–Smale model with topological interactions is studied.
• Kinetic and hydrodynamic descriptions of the topological model are derived.
• The notion of topological distance is extended to other models of collective motion.

We introduce a Cucker–Smale-type model for flocking, where the strength of interaction between two agents depends on their relative separation (called “topological distance” in previous works), which is the number of intermediate individuals separating them. This makes the model scale-free and is motivated by recent extensive observations of starling flocks, suggesting that the interaction ruling animal collective behavior depends on topological rather than the metric distance. We study the conditions leading to asymptotic flocking in the topological model, defined as the convergence of the agents’ velocities to a common vector. The shift from metric to topological interactions requires development of new analytical methods, taking into account the graph-theoretical nature of the problem. Moreover, we provide a rigorous derivation of the mean-field limit of large populations, recovering kinetic and hydrodynamic descriptions. In particular, we introduce the novel concept of relative separation in continuum descriptions, which is applicable to a broad variety of models of collective behavior. As an example, we shortly discuss a topological modification of the attraction–repulsion model and illustrate with numerical simulations that the modified model produces interesting new pattern dynamics.