@article{SF_1992__44_1_135161,
     author = {G. Markowsky},
     title = {Ordering {D-classes} and computing {Schein} rank is hard.},
     journal = {Semigroup forum},
     pages = {373--375},
     publisher = {mathdoc},
     volume = {44},
     number = {1},
     year = {1992},
     zbl = {0760.20018},
     url = {http://geodesic.mathdoc.fr/item/SF_1992__44_1_135161/}
}