ESLAMI, E., & STEWART, P. (1992). TWO-SIDED ESSENTIAL NILPOTENCE. International Journal of Mathematics and Mathematical Sciences, 1992(), 351-354. https://doi.org/10.1155/S0161171292000449
Xu, J. (2011). Resource and agency [doctoral dissertation, The University of Hong Kong]. Airiti Library. https://www.airitilibrary.com/Article/Detail?DocID=U0029-1812201200018917