Relationships Between Two Definitions of Fading Memory for Discrete-Time Systems

In this paper, we refer to two definitions of fading memory property, which were published in the literature, for discrete-time circuits and systems. One of these definitions relates to systems working with signals (sequences) defined for both the positive and negative integers, expanding from minus infinity to plus infinity. On the other hand, the second one refers to systems processing sequences defined only for nonnegative integers, that is starting at the discrete-time point equal to zero and expanding to plus infinity. We show here that the second definition follows from the first one. That is they are not independent. Moreover, we also show that if an operator describing a system possesses a fading memory according to the second definition, then its associated operator has this property, too, but in accordance with the first definition.