@article{PIM_1981_N_S_29_43_a1,
     author = {Stojan Bogdanovi\'c},
     title = {Qr-semigroups},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {15 },
     publisher = {mathdoc},
     volume = {_N_S_29},
     number = {43},
     year = {1981},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PIM_1981_N_S_29_43_a1/}
}