@article{SM_1967_3_1_a2,
     author = {E. S. Lyapin},
     title = {Semigroups of directed transformations of ordered sets},
     journal = {Sbornik. Mathematics},
     pages = {33--39},
     publisher = {mathdoc},
     volume = {3},
     number = {1},
     year = {1967},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1967_3_1_a2/}
}