A Sufficient and Necessary Condition for Subellipticity of Left Invariant Differential Operators on Nilpotent Left Group G2n+k
郑驻军, 张志平
A Sufficient and Necessary Condition for Subellipticity of Left Invariant Differential Operators on Nilpotent Left Group G2n+k
Zheng Zhujun, Zhang Zhiping
数学季刊 . 1995, (2): 81 -84 .