Distributed Nonlinear Observer with Robust Performance - A Circle   Criterion Approach
  In this paper, we present a distributed version of the KYP-Lemma with the goal to express the strictly positive real-property for a class of physically interconnected systems by a set of local LMI-conditions. The resulting conditions are subsequently used to constructively design distributed circle criterion estimators, which are able to collectively estimate an underlying linear system with a sector bounded nonlinearity. 
Jingbo Wu, Frank Allg\"ower
cs.SY