Similarly, for τ one gets d τ τ = ∑ ∑ i = 1 n ∂ ∂ ∂ ∂ u i R R 2 + | u | 2 R 2 + | u | 2 d u i + ∂ ∂ ∂ ∂ τ τ R R 2 + | u | 2 R 2 + | u | 2 d τ τ = ∑ ∑ i = 1 n R 4 4 R 2 u i d u i ( R 2 − − | u | 2 ) , yielding − − d τ τ 2 = − − ( R 4 R 4 ( u ⋅ ⋅ d u ) ( R 2 − − | u | 2 ) 2 ) 2 = − − R 2 16 R 4 ( u ⋅ ⋅ d u ) 2 ( R 2 − − | u | 2 ) 4 .