We analyze the stability and stabilizability properties of mixed retarded-neutral typesystems when the neutral term may be singular. We consider an operator differentialequation model of the system in a Hilbert space, and we are interested in the criticalcase when there is a sequence of eigenvalues with real parts converging to zero. In thiscase, the system cannot be exponentially stable, and we study conditions under which itwill be strongly stable. The behavior of spectra of mixed retarded-neutral type systemsprevents the direct application of retarded system methods and the approach of pureneutral type systems for the analysis of stability. In this paper, two techniques arecombined to obtain the conditions of asymptotic non-exponential stability: the existenceof a Riesz basis of invariant finite-dimensional subspaces and the boundedness of theresolvent in some subspaces of a special decomposition of the state space. For unstablesystems, the techniques introduced enable the concept of regular strong stabilizabilityfor mixed retarded-neutral type systems to be analyzed.

On an arbitrary reflexive Banach space, we build asymptotic observers for an abstract class of nonlinear control systems with possible compact outputs. An important part of this paper is devoted to various examples, where we discuss the existence of persistent inputs which make the system observable. These results make a wide generalization to a nonlinear framework of previous works on the observation problem in infinite dimension (see [11,18,22,26,27,38,40] and other references therein).