equation icon  a' b' a' b' g'[ ( d) 2 ] Uaba'b';gu u = g (ag b)gg -r Ra'0g'0 + Ra'0g'd_O_ ub'+ O(r ) , (502) U '' 'ua'ub'ug'= ga' gb' [rR ' _O_du '+ O(r2)], (503) abab ;g (a b) a0d0 b