Quaternions Defined by Matrices

Theorem
Let $\mathbf 1, \mathbf i, \mathbf j, \mathbf k$ denote the following four elements of the matrix space $\map {\MM_\C} 2$:


 * $\mathbf 1 = \begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}

\qquad \mathbf i = \begin{bmatrix} i & 0 \\ 0 & -i \end{bmatrix} \qquad \mathbf j = \begin{bmatrix} 0 & 1 \\ -1 & 0 \end{bmatrix} \qquad \mathbf k = \begin{bmatrix} 0 & i \\ i & 0 \end{bmatrix} $ where $\C$ is the set of complex numbers.

Then $\mathbf 1, \mathbf i, \mathbf j, \mathbf k$ are related to each other in the following way:

Proof
This is demonstrated by straightforward application of conventional matrix multiplication: