One computes according to the standard rules for computing differentials (though one is really computing the rigorously defined exterior derivatives), d x 1 ( u ) = d ( 2 R 2 u 1 R 2 − − | u | 2 ) = ∂ ∂ ∂ ∂ u 1 2 R 2 u 1 R 2 − − | u | 2 d u 1 + ⋯ ⋯ + ∂ ∂ ∂ ∂ u n 2 R 2 u 1 R 2 − − | u | 2 d u n + ∂ ∂ ∂ ∂ τ τ 2 R 2 u 1 R 2 − − | u | 2 d τ τ , ⋮ ⋮ d x n ( u ) = d ( 2 R 2 u n R 2 − − | u | 2 ) = ⋯ ⋯ , d τ τ ( u ) = d ( R R 2 + | u | 2 R 2 − − | u | 2 ) = ⋯ ⋯ , and substitutes the results into the right hand side. This yields ( σ σ − − 1 ) ∗ ∗ h R 1 ( n ) = 4 R 2 [ ( d u 1 ) 2 + ⋯ ⋯ + ( d u n ) 2 ] ( R 2 − − | u | 2 ) 2 ≡ ≡ h R 2 ( n ) .