Algorithms for Coxianization of Phase-Type Generators

This paper develops algorithms for finding Coxian generators to phase-type (PH)-majorize a PH-generator T with only real eigenvalues. In the first part of this paper, we investigate matrices S and P satisfying TP = PS and Pe = e. Conditions on T are identified for S to be an ordered Coxian generator and for P to be nonnegative, which consequently implies that S PH-majorizes T. It is shown that every PH-generator with only real eigenvalues is PH-majorized by some Coxian generator. In the second part of this paper, the results on S and P and the conditions on T are used to develop efficient algorithms for Coxianization of PH-generators. Numerical examples are presented for a comparison between the developed algorithms.