  Extended time-delay auto-synchronization (ETDAS) is a promising technique for
stabilizing unstable periodic orbits in low-dimensional dynamical systems. The
technique involves continuous feedback of signals delayed by multiples of the
orbit's period in a manner that is especially well-suited for fast systems and
optical implementation. We show how to analyze the stability of a given
implementation of ETDAS without explicit integration of time-delay equations.
To illustrate the method and point out some nontrivial features of ETDAS, we
obtain the domain of control for a period-one orbit of the driven, damped
pendulum.
