@article{SF_1991__42_3_135062,
     author = {J. Almeida},
     title = {On the membership problem for pseudo-varieties of commutative semigroups.},
     journal = {Semigroup forum},
     pages = {47--52},
     publisher = {mathdoc},
     volume = {42},
     number = {3},
     year = {1991},
     zbl = {0728.20050},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__42_3_135062/}
}