Topology
《Topology》 · 第 19 页 · Chapter 2，Section 2.4；印刷第 70 页，Chapter 5，Section 5.5
For each $\lambda \in k, \lambda \neq 0$ there is an (infinite dimensional) $U$-module $M(\lambda)$ with basis $m_0, m_1, m_2, \dots$... we can take $M(\lambda) = U / (U E + U (K - \lambda))$. The description makes it clear that $M(\lambda)$ has the following universal property: If $M$ is a $U$-module and $m \in M$ a vector with $E m = 0$ and $K m = \lambda m$, then there is a unique homomorphism of $U$-modules $\varphi \colon M(\lambda) \to M$ with $\varphi(m_0) = m$.