A method to prove that nonlinear models can be unidentifiable

Some authors have questioned the very interest of developing methods to test nonlinear models for identifiability, and conjectured that all nonlinear models are either identifiable or trivially unidentifiable. The purpose of this short paper is two fold: first to present a method that can be used to prove that some nonlinear models are unidentifiable and second to exhibit nontrivial examples of such models.