Root and pre-constant signals of the 1D Teager-Kaiser operator

We give an input/output characterization of the Teager-Kaiser operator, then we classify the root signals (those that remain invariant) and the pre-constant signals (those that produce a constant output) of the operator. With the gained knowledge, a simple detector of sinusoidal signals and of their corresponding frequencies is designed. The methods used allow for a certain amount of generalization providing a paradigm of sorts for the design of quadratic and other polynomial filters.