摘要：Monounireducible nonhomogeneous semi-
Markov processes are defined and investigated. The mono-
unireducible topological structure is a sufficient condition
that guarantees the absorption of the semi-Markov process
in a state of the process. This situation is of fundamental
importance in the modelling of credit rating migrations because permits the derivation of the distribution function of
the time of default. An application in credit rating modelling is given in order to illustrate the results.
