@article{SF_1987__36_134903,
     author = {P. Normak},
     title = {On equalizer-flat and pullback-flat acts.},
     journal = {Semigroup forum},
     pages = {293--314},
     publisher = {mathdoc},
     volume = {36},
     year = {1987},
     zbl = {0625.20058},
     url = {http://geodesic.mathdoc.fr/item/SF_1987__36_134903/}
}