@article{ADM_2019_28_2_a12,
     author = {Behnaz Tolue and Parvaneh Zolfaghari and Gholamreza Moghaddasi},
     title = {Free ultra-groups, generators and relations},
     journal = {Algebra and discrete mathematics},
     pages = {308--316},
     publisher = {mathdoc},
     volume = {28},
     number = {2},
     year = {2019},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ADM_2019_28_2_a12/}
}