@article{SF_1986__34_134846,
     author = {A. Varisco and A. Cherubini},
     title = {On {Ek-semigroups.}},
     journal = {Semigroup forum},
     pages = {305--320},
     publisher = {mathdoc},
     volume = {34},
     year = {1986},
     zbl = {0623.20049},
     url = {http://geodesic.mathdoc.fr/item/SF_1986__34_134846/}
}