Looking at the previous equation, we must have H ̸ ≡ ±m as well. The m and H 1 containing matrices are 2 × 2. The outer product ⊗ produces a 1 4×4 matrix. I.e. in and ( ), the 2 × 2 unit matrix 1 = 1 2×2 representation is there to make it a 4 × 4 matrix.