Class Field Theory
《Class Field Theory》 · 印刷第 7—8 页 Section 1.2.2「Unitary and Orthogonal Groups」，正交矩阵与特殊正交群定义；本段为实数域上的情形
In a similar fashion, an $n\times n$ real matrix $A$ is said to be orthogonal if the column vectors of $A$ are orthonormal. […] The set of $n\times n$ orthogonal matrices with determinant one is the special orthogonal group, denoted $SO(n)$.