Class Field Theory
《Class Field Theory》 · 印刷第 6—7 页 Section 1.2.2「Unitary and Orthogonal Groups」，式 (1.2) 及酉群命名句；本书此处为复数域上正定内积的情形
An $n\times n$ complex matrix $A$ is said to be unitary if the column vectors of $A$ are orthonormal, that is, if $\sum_{l=1}^n\overline{A_{lj}}A_{lk}=\delta_{jk}$. […] Thus, the collection of unitary matrices is a subgroup of $GL(n;\mathbb C)$. We call this group the unitary group and we denote it by $U(n)$.