equation icon  { 1 3 1 1 1 } s = r 1 - -raa(u)_O_a + -r2[aa(u)_O_a]2 - --r2a0(u)- -r2aa(u)_O_a - -r2Ra0b0(u)_O_a_O_b + O(r3) 2 8 8 6 6 (220)