K3=f(xn-1+h,yn-1+h(2K2-K1) f(xn-1y-1)+(xn1,y21)+h(2K2-K1)(xn1,yn=1) +[hf(xn1yn-1)+h2(2K2-K1)fm(xn1y-1)+ h(2K2-kfln-1ym-1]+O(h =y(xn-1)+hy"(xn-1)+h(2K2-2K1)fy(xn-1yn-1) h x20(x1)+(2K2-2K1)(xn1y)+ (4K2-4K1K2)f"(xn-1y-)+0(h) h y(xn-1)+hy(xn1)+y"(xn-1)+O(h3)( , (2 )) 3 1 1 K2 K1 K f x h y h = n- + n- + - ( , ) ( , ) (2 ) ( , ) -1 -1 -1 -1 2 1 -1 -1 = + ¢ + - ¢ n n x n n y n n f x y hf x y h K K f x y (2 ) ( , )] ( ) [ ( , ) (2 ) ( , ) 2! 1 3 1 1 2 2 1 2 2 1 1 1 2 1 1 2 h K K f x y O h h f x y h K K f x y y n n x n n xy n n - ¢¢ + + ¢¢ + - ¢¢ + - - - - - - ( ) ( ) (2 2 ) ( , ) -1 -1 2 1 -1 -1 = ¢ + ¢¢ + - ¢ n n y n n y x hy x h K K f x y (4 4 ) ( , )] ( ) [ ( ) (2 2 ) ( , ) 2 3 2 1 2 1 1 1 2 1 1 1 2 K K K f x y O h y x K K f x y h y n n n xy n n - ¢¢ + + ¢¢¢ + - ¢¢ + - - - - - ( ) ( ) 2 ( ) ( ) 3 1 2 1 1 y x O h h y x hy x = ¢ n- + ¢¢ n- + ¢¢¢ n- +