Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 63 页 · Section 3.1「Series and Composition Series」
A subgroup which is a term of at least one $\Omega$-series is said to be $\Omega$-subnormal in $G$. Thus $H$ is $\Omega$-subnormal in $G$ if and only if there exist distinct $\Omega$-subgroups $H_0=H,H_1,\ldots,H_n=G$ such that $H=H_0\trianglelefteq H_1\trianglelefteq\cdots\trianglelefteq H_n=G$. When $\Omega$ is empty, we shall simply speak of a series and a subnormal subgroup.