(77) F*f* =| -S₁₁ - L, -S₁₂, -S₁₃. -S₁₄ |
| -S₂₁, -S₂₂ - L, -S₂₃, -S₂₄ |
| -S₃₁ -S₃₂, -S₃₃ - L, -S₃₄ |
| -S₄₁ -S₄₂ -S₄₃ -S₄₄ - L |
and hence, we can put
(78) fF = S - L, F*f* = -S - L,
where by L, we mean L-times the unit matrix, i.e. the matrix with elements
| Lehk |, (ehh = 1, ehk = 0, h ≠ k h, k = 1, 2, 3, 4).
Since here SL = LS, we deduce that,
F*f*fF = (-S - L)(S - L) = -SS + L²,