@article{CMUC_1990_31_2_a24,
     author = {Pol\'ak, Jaroslav},
     title = {Aggregation and disaggregation in {Markov} chains},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {403--404},
     year = {1990},
     volume = {31},
     number = {2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1990_31_2_a24/}
}