#
Intrinsic difficulties in using
the doubly-infinite time axis
for input-output systems theory.

## T.T. Georgiou and M.C. Smith

## Abstract

We point out
that the natural definitions of stability and causality in
input-output systems theory lead to certain inconsistencies
when inputs and outputs are allowed to have support on the
doubly-infinite time-axis.
In contrast, the use of the semi-infinite time axis gives
a framework which appears to be more satisfactory to the
intuition.