@article{CMUC_1963_4_1_a3,
     author = {Baayen, Cor P. and Hedrl{\'\i}n, Zden\v{e}k},
     title = {On the product and sum of a system of transformation semigroups},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {29--42},
     year = {1963},
     volume = {4},
     number = {1},
     mrnumber = {0159895},
     zbl = {0116.25601},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1963_4_1_a3/}
}