are of much importance. Let us write,
(70) fF =| S₁₁ - L S₁₂ S₁₃ S₁₄ |
| S₂₁ S₂₂ - L S₂₃ S₂₄ |
| S₃₁ S₃₂ S₃₃ - L S₃₄ |
| S₄₁ S₄₂ S₄₃ S₄₄ - L |
Then (71) S₁₁ + S₂₂ + S₃₃ + S₄₄ = 0.
Let L now denote the symmetrical combination of the indices 1, 2, 3, 4, given by
(72) L = ½(f₂₃ F₂₃ + f₃₁F₃₁ + f₁₂ + F₁₂ + f₁₄ F₁₄
+ f₂₄ F₂₄ + f₃₄ F₃₄)
Then we shall have