I. Introduction
In recent years, consensus and synchronization problems have received increasing attention in the control area due to their relevance in many engineering applications (see [5] and references therein). Consensus problems have been considered for identical linear systems in [22] and then extended to the more general nonlinear case in [1] and [33]. Much work has also been produced toward the problem of synchronization of nonidentical systems (see, e.g., the recent works [19], [26] and references therein). Here, we follow the paradigm of [31], where heterogeneity is taken into account via the presence of an property (actually, an gain property because we address nonlinear saturation phenomena) from a disturbance input to a performance output. Then, small-gain results can be applied whenever the mismatch among the dynamics can be described in terms of an bounded perturbation.