equation icon  ( ) ( ) du = e0adxa - _O_a ebadxa , (164) [ ]( ) d^xa = - raa - rwab_O_b + 1-r2Sa + O(r3) e0a dxa 2 [ ( 1 ) 1 ]( ) + dab + raa - rwac_O_c + --r2Sa _O_b + -r2Sab + O(r3) eba dxa . (165) 3 6