find the points of differentiability of f(z) where : 1) f(z) =  4 x{^3}+ iy{^3} 2) f(z) =   x{^2} + y{^2}+ i2xy  3) f(z) =   xy + x{^2}yi 4) f(z) =   x{^3} + 3xy{^2}-3x+ i(y{^3}+3x{^2}y-3y)    my solutions : 1) nowhere analytic and diff at y=  \pm 2x 2)nowhere analytic and diff at y=0 (x Axis) 3)nowhere analytic 4) x=0 or y=0