When classical nonlinear time-delay state-space systems admit an input-output equation of neutral type

Paper in proceeding, 2010

This paper deals with nonlinear retarded time-delay systems that surprisingly admit an input-output representation of neutral type. It is shown that such an unexpected behaviour represents a strictly nonlinear phenomenon, for it cannot happen in the linear time-delay case where retarded systems always admit an input-output representation of retarded type. A necessary and sufficient condition under which the nonlinear systems admit a neutral input-output representation is given and strategies for finding an inputoutput representation of retarded type are briefly outlined. Some open problems that arise, like minimality and system transformations, are discussed as well.