With this one may write d x 1 ( u ) = 2 R 2 ( R 2 − − | u | 2 ) d u 1 + 4 R 2 u 1 ( u ⋅ ⋅ d u ) ( R 2 − − | u | 2 ) 2 , from which ( d x 1 ( u ) ) 2 = 4 R 2 ( r 2 − − | u | 2 ) 2 ( d u 1 ) 2 + 16 R 4 ( R 2 − − | u | 2 ) ( u ⋅ ⋅ d u ) u 1 d u 1 + 16 R 4 ( u 1 ) 2 ( u ⋅ ⋅ d u ) 2 ( R 2 − − | u | 2 ) 4 .