@article{JIS_2011__14_7_a7,
     author = {Umar,  A.},
     title = {Combinatorial results for semigroups of orientation-preserving partial transformations.},
     journal = {Journal of integer sequences},
     year = {2011},
     volume = {14},
     number = {7},
     zbl = {1226.20052},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/JIS_2011__14_7_a7/}
}