[; S_{\lambda_1,\lambda_2}(\underline{v}) = \underline{\underline{S}}_{\lambda_1,\lambda_2} \underline{v} ;]
Linear![; \underline{\underline{R}}_{\theta} = \begin{pmatrix} \lambda_1 & 0\\ 0 & \lambda_2\\ \end{pmatrix} ;]
[; \underline{\underline{S}}_{\lambda_1,\lambda_2,\theta} = \underline{\underline{R}}_{\theta}~ \underline{\underline{S}}_{\lambda_1,\lambda_2}~ \underline{\underline{R}}_{-\theta} = \begin{pmatrix} \lambda_1 \cos^2{\theta}+\lambda_2 \sin^2{\theta} & -(\lambda_1-\lambda_2) \cos{\theta} \sin{\theta} \\ (\lambda_1-\lambda_2) \cos{\theta} \sin{\theta} & \lambda_1 \cos^2{\theta}+\lambda_2 \sin^2{\theta} \\ \end{pmatrix} ;]