Sufficiency-type stability and stabilization criteria for linear time-invariant systems with constant point delays

A set of criteria of asymptotic stability for linear and time-invariant systems with constant point delays are derived. The criteria are concerned with alpha-stability local in the delays and epsilon-stability independent of the delays, namely, stability with all the characteristic roots in Re s less than or equal to -alpha < 0 for all delays in some defined real intervals including zero and stability with characteristic roots in Re s < -epsilon < 0 as epsilon --> 0(+) for all possible values of the delays, respectively. The results are classified in several groups according to the technique dealt with. The used techniques include both Lyapunov's matrix inequalities and equalities and Gerschgorin's circle theorem. The Lyapunov's inequalities are guaranteed if a set of matrices, built from the matrices of undelayed and delayed dynamics, are stability matrices. Some extensions to robust stability of the above results are also discussed.