@article{SF_1985__31_134740,
     author = {K. Todorov and B. Bijev},
     title = {Idempotent-generated subsemigroups of the symmetric semigroup of degree four: computer investigations.},
     journal = {Semigroup forum},
     pages = {119--122},
     publisher = {mathdoc},
     volume = {31},
     year = {1985},
     zbl = {0565.20047},
     url = {http://geodesic.mathdoc.fr/item/SF_1985__31_134740/}
}