Collecting terms containing various powers of λ, we get λ λ 3 ( I 0 1 − − ∂ ∂ I 1 ∂ ∂ A 1 − − A T ⋅ ⋅ ∂ ∂ I 0 ∂ ∂ A ) + λ λ 2 ( I 1 1 − − ∂ ∂ I 2 ∂ ∂ A 1 − − A T ⋅ ⋅ ∂ ∂ I 1 ∂ ∂ A ) + λ λ ( I 2 1 − − ∂ ∂ I 3 ∂ ∂ A 1 − − A T ⋅ ⋅ ∂ ∂ I 2 ∂ ∂ A ) + ( I 3 1 − − ∂ ∂ I 4 ∂ ∂ A 1 − − A T ⋅ ⋅ ∂ ∂ I 3 ∂ ∂ A ) = 0 .