Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 345 页 · Section 11.4「Group-Theoretic Interpretations of the (Co)homology Groups」，小节「Central Extensions」
An extension $C\overset{\mu}{\rightarrowtail}E\twoheadrightarrow G$ is called central if $\operatorname{Im}\mu$ is contained in the centre of $E$. In this case $G$ operates trivially on $C$, so that $C$ is a trivial $G$-module.