@article{SF_1991__42_3_135087,
     author = {M.V. Sapir},
     title = {On cross semigroup varieties and related questions.},
     journal = {Semigroup forum},
     pages = {345--364},
     year = {1991},
     volume = {42},
     number = {3},
     zbl = {0741.20042},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__42_3_135087/}
}