1 刘仲奎. 强忠实右S - 系[ J ] . 数学杂志, 1995, 15( 4) : 429~ 435.
2 刘仲奎. 半群的S - 系理论[M] . 北京: 科学出版社, 1999.
3 Liu Zhong kui. A char acterization of regular monoids by flatness of left acts[ J] . Semigroup Fourm, 1993, 46: 85~ 89.
4 Bulman- Fleming S. Flat and strong ly flat S - systems[ J] . Communicatio n in Alg ebra, 1992, 20 ( 9) : 2 553~ 2 567.
5 Normak P. On equalizer- flat and pullback- flat acts[ J] . Semigroup Forum, 1987, 36: 293~ 313.