[; P(\underline{v}) = \underline{\underline{P}}~ \underline{v} ;]
[; \underline{\underline{P}} = \begin{pmatrix} 1 & 0\\ 0 & \0\\ \end{pmatrix} = \underline{\underline{S}}_{1,0} ;]
[; \underline{\underline{P}}_1 = \begin{pmatrix} 0 & 0\\ 0 & 1\\ \end{pmatrix} = \underline{\underline{S}}_{0,1} ;]
[; \underline{\underline{P}}_{\theta} = \underline{\underline{R}}_{\theta}~ \underline{\underline{P}}~ \underline{\underline{R}}_{-\theta} = \begin{pmatrix} \cos^2{\theta} & -\cos{\theta} \sin{\theta} \\ \cos{\theta} \sin{\theta} & \sin^2{\theta} \\ \end{pmatrix} ;]
[; \underline{\underline{P}}_{\theta}^{} = \underline{\underline{P}}_{\theta}, \quad n>1 ;]