@article{SF_1979__18_134361,
     author = {A. Varisco and Spoletini A. Cherubini},
     title = {On {Putcha's} {Q-semigroups.}},
     journal = {Semigroup forum},
     pages = {313--318},
     publisher = {mathdoc},
     volume = {18},
     year = {1979},
     zbl = {0429.20051},
     url = {http://geodesic.mathdoc.fr/item/SF_1979__18_134361/}
}