Topology
《Topology》 · 第 121 页 · Chapter 6，Section 6.22
We want to apply this to matrix coefficients: Let $M$ be a finite dimensional $U$-module. For all $m \in M$ and $f \in M^*$ let $c_{f,m} \in U^*$ be the linear form with $c_{f,m}(u) = f(um)$ for all $u \in U$.