@article{SF_1993__46_1_135214,
     author = {Z. Liu},
     title = {A characterization of regular monoids by flatness of left acts.},
     journal = {Semigroup forum},
     pages = {85--89},
     publisher = {mathdoc},
     volume = {46},
     number = {1},
     year = {1993},
     zbl = {0766.20024},
     url = {http://geodesic.mathdoc.fr/item/SF_1993__46_1_135214/}
}