Then, invoking the arbitrariness of λ, we have I 0 1 − − ∂ ∂ I 1 ∂ ∂ A 1 − − A T ⋅ ⋅ ∂ ∂ I 0 ∂ ∂ A = 0 I 1 1 − − ∂ ∂ I 2 ∂ ∂ A 1 − − I 2 1 − − ∂ ∂ I 3 ∂ ∂ A 1 − − A T ⋅ ⋅ ∂ ∂ I 2 ∂ ∂ A = 0 I 3 1 − − ∂ ∂ I 4 ∂ ∂ A 1 − − A T ⋅ ⋅ ∂ ∂ I 3 ∂ ∂ A = 0 .