equation icon  F abcd(x,y) = \~/ a \~/ c G+ (x,y) \~/ b \~/ d G+ (x,y) + \~/ a \~/ d G+ (x,y) \~/ b \~/ c G+ (x,y) n x y (n x y n ) x y n( x y n ) +2Dabx \~/ cyG+n(x,y) \~/ dyG+n(x,y) + 2Dcdy \~/ axG+n(x,y) \~/ bxG+n(x, y) ab cd( +2 ) +2D x Dy Gn (x, y) , (38)