Definition:Group of Rotation Matrices Order 4

Definition
Consider the algebraic structure $S$ of rotation matrices:
 * $R_4 = \left\{ {\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}, \begin{bmatrix} 0 & 1 \\ -1 & 0 \end{bmatrix}, \begin{bmatrix} -1 & 0 \\ 0 & -1 \end{bmatrix}, \begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix} }\right\}$

under the operation of (conventional) matrix multiplication.

$R_4$ is the group of rotation matrices of order $4$.

Also see

 * Rotation Matrices Order 4 form Group