One has from the formula for σ V x j = V i ∂ ∂ ∂ ∂ u i ( 2 R 2 u j R 2 − − | u | 2 ) = 2 R 2 V j R 2 − − | u | 2 − − 4 R 2 u j ⟨ ⟨ V , u ⟩ ⟩ ( R 2 − − | u | 2 ) 2 , ( here V | u | 2 = 2 ∑ ∑ k = 1 n V k u k ≡ ≡ 2 ⟨ ⟨ V , u ⟩ ⟩ ) V τ τ = V ( R R 2 + | u | 2 R 2 − − | u | 2 ) = 4 R 3 ⟨ ⟨ V , u ⟩ ⟩ ( R 2 − − | u | 2 ) 2 .