A class of neutral-type delay differential equations that are effectively retarded

We demonstrate that some delay-differential equations of neutral type are, up to basis transformation, equivalent to retarded delay differential equations. In particular, for two classes of neutral delay differential equation models, we use state transformations to show that delayed derivatives can in some cases be expressed in terms of the model´s state. Hence, we obtain conditions when neutral delay differential equations can be transformed into retarded delay differential equations.