Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 214 页 · Chapter 8，Section 8.1「Representations and Modules」，小节「Group Rings and Group Algebras」
If $G$ is a group and $R$ is any ring with an identity element, the group ring $RG$ is defined to be the set of all formal sums $\sum_{x\in G}r_xx$ where $r_x\in R$ and $r_x=0$ with finitely many exceptions, together with the rules of addition and multiplication $\sum r_xx+\sum r'_xx=\sum(r_x+r'_x)x$ and $(\sum r_xx)(\sum r'_xx)=\sum_x(\sum_{yz=x}r_yr'_z)x$.