@article{10_21136_CMJ_2018_0343_17,
     author = {Umar, Abdullahi},
     title = {Presentations for subsemigroups of $PD_n$},
     journal = {Czechoslovak Mathematical Journal},
     pages = {417--426},
     publisher = {mathdoc},
     volume = {69},
     number = {2},
     year = {2019},
     doi = {10.21136/CMJ.2018.0343-17},
     mrnumber = {3959954},
     zbl = {07088794},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.21136/CMJ.2018.0343-17/}
}