Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 80 页 · Section 3.3「Direct Decompositions」
Let $G$ be a group with operator domain $\Omega$. An $\Omega$-subgroup $H$ is called an $\Omega$-direct factor of $G$ if there exists an $\Omega$-subgroup $K$ such that $G=H\times K$; then $K$ is called an $\Omega$-direct complement of $H$ in $G$. If there are no proper nontrivial $\Omega$-direct factors of $G$, then $G$ is said to be $\Omega$-indecomposable (or just indecomposable if $\Omega=\emptyset$).