We analyze the collective behavior of relatively large networks with the characteristic that nodes may be compartmentalized in modules. These are groups of systems that may be regarded as subnetworks in which each group of nodes achieves consensus with a certain rapidity. The dynamics of such networks exhibit, at least, two time-scales, for which singular perturbation methods may be used to assess the overall behavior. In this paper, we demonstrate that there are actually three natural time-scales and, accordingly, three interconnected dynamical systems with distinct speeds of convergence coexist. Our main statement establishes conditions for global asymptotic stability of the origin in such networked systems. In particular, we focus on bilinear heterogeneous systems and provide an illustrative example concerning chaotic oscillators.
I. INTRODUCTIONModular networks are, generally speaking, networks of a large number of nodes that are compartmentalised in groups of nodes called modules. These are sparsely connected among themselves and the nodes contained within are densely interconnected [1], [2], [3]. Because of their complexity and large dimension, they are difficult to analyze; one approach consists in considering many agents in a module as a single node. This approach has been used both for undirected [4], [5], [6] and directed networks [7], [8]. This view is specially fit in networks such that agents within a module reach consensus relatively fast, before achieving consensus with nodes of other modules. Or, in other words, modules that have reached their own individual consensus find agreement among themselves at a slower pace [6], [9]. This is specially true if the coupling gain within a module is considerably larger than the one amongst modules.Such considerations lead naturally to multi-time scale models that can be studied using singular-perturbation theory [10], using the inverse of the coupling gain as singular parameter [11]. In the latter, is used to define a two-time scale modelling of a heterogeneous network ; the authors show that the emergent dynamics correspond to a slow subsystem while the synchronization errors form a fast subsystem. Singular perturbation is also used in [12] to show that, for interconnected linear systems with switching interconnection topology and linear coupling, if the coupling gain is sufficiently high, the synchronized behavior can be approximated by a reduced order switching system.