Linear Algebra Done Right
《Linear Algebra Done Right》 · 第 109 页 · Section 3F「Duality」，Definition 3.121
For $U\subseteq V$, the annihilator of $U$, denoted by $U^0$, is defined by $U^0=\{\varphi\in V':\varphi(u)=0\text{ for all }u\in U\}$.