Iterative methods and convergence for the time-delay Lyapunov equation

Sammanfattning: The delay Lyapunov equation is a matrix boundary value problem arising in the characterization of many properties of time-delay systems, for example stability analysis. Its numerical treatment is challenging. For the special case of single-delay systems, a new algorithm based on a delay free formulation has recently been proposed. Using this formulation it is possible to obtain a linear system of equations with an equivalent solution. This linear system can be solved with GMRES or a similar iterative method, thus allowing to efficiently solve large-scale problems. In addition to the preconditioner proposed in the literature, on the basis of solving a T-Sylvester equation, a new preconditioner for this iterative method is derived here. It uses the diagonals of the time-delay system’s n × n state matrices to compute an approximation of the action of the n² × n² matrix associated to the linear system. Computational cost and convergence of this new preconditioner are investigated and proved. Examples for its efficiency under certain conditions are given and it is compared to the preconditioner from the literature. A pseudospectral analysis of the corresponding operators is conducted to get a better understanding of the convergence of both preconditioners. Several ways to obtain pseudospectra based convergence estimates are presented and their descriptiveness for different types of problems is discussed.