Jacob Kogan and A. Leizarowitz, "Frequency domain criterion
for robust stability of interval time-delay systems", 
<I> Automatica</I>, 1995, Vol. 31, No. 3, pp. 463-469. 
<P>

Stability conditions for time-delay systems are of great importance
for industrial applications. Delays often occur in the transmission of
information or material between different parts of a system.
Transportation systems, communication systems, chemical processing
systems, metallurgical processing systems, environmental systems, and
power systems are examples of time-delay systems. The mathematical
formulation of a time-delay system results in a system of
delay-differential equations. Any mathematical model of an engineering
system possesses the unavoidable inaccuracy. The existence of the
inaccuracies implies that the analysis of stability and performance as
well as system design, based on a nominal model only, may not be
meaningful in applications. 
<P>
In this paper, a new criterion is described for robust stability of
interval time-delay systems.  
<P> From the authors' summary