SIAM JOURNAL ON SCIENTIFIC COMPUTING, VOL.26, NO.4, PP.1289-1313, 2005. TITLE: Block SOR Preconditioned Projection Methods for Kronecker Structured Markovian Representations AUTHORS: Peter Buchholz and Tugrul Dayar ABSTRACT: Kronecker structured representations are used to cope with the state space explosion problem in Markovian modeling and analysis. Currently, an open research problem is that of devising strong preconditioners to be used with projection methods for the computation of the stationary vector of Markov chains (MCs) underlying such representations. This paper proposes a block successive overrelaxation (BSOR) preconditioner for hierarchical Markovian models (HMMs; throughout the paper, the HMM acronym stands for hierarchical Markovian models and should not be confused with the HMM that is sometimes used for hidden Markov models.) that are composed of multiple low-level models and a high-level model that defines the interaction among low-level models. The Kronecker structure of an HMM yields nested block partitionings in its underlying continuous-time MC which may be used in the BSOR preconditioner. The computation of the BSOR preconditioned residual in each iteration of a preconditioned projection method becomes the problem of solving multiple nonsingular linear systems whose coefficient matrices are the diagonal blocks of the chosen partitioning. The proposed BSOR preconditioner solves these systems using sparse LU or real Schur factors of diagonal blocks. The fill-in of sparse LU factorized diagonal blocks is reduced using the column approximate minimum degree (COLAMD) ordering. A set of numerical experiments is presented to show the merits of the proposed BSOR preconditioner. KEY WORDS: Markov chains, Kronecker structured numerical techniques, block SOR, preconditioning, projection methods, real Schur factorization, COLAMD ordering