equation icon  @ u = e0 - _O_ ea, (166) a a[ a a ] a a a b 1- 2 a 3 0 @a^x = - ra - rw b_O_ + 2r S + O(r ) ea [ ( ) ] + dab + raa - rwac_O_c + 1-r2Sa _O_b + 1r2Sab + O(r3) eba. (167) 3 6