中国科学院数学与系统科学研究院期刊网
Some Homological Characterizations of Semigroups and Semirings
Javed AHSAN, Zhong Kui LIU, Muhammad SHABIR
Acta Mathematica Sinica . 2011, (10): 2065 -2072 .  DOI: 10.1007/s10114-011-8180-5