@article{NSJOM_1980_10_Nonea_17,
     author = {Bogdanovi\'c Stojan},
     title = {r- 

 semigrupe},
     journal = {Novi Sad Journal of Mathematics},
     pages = {149-152},
     publisher = {mathdoc},
     volume = {10},
     year = {1980},
     url = {http://geodesic.mathdoc.fr/item/NSJOM_1980_10_Nonea_17/}
}