Examining a problem I find that: 
$E[(Y\_n)^3] = \sum\_{i=1}^nE[(X\_i)^3]$. Where $Y\_n=\sum\_{i=1}^nX\_i$.

Now, I'm not able to understanding the following step:
$$\phi^{'''}\_{Y\_n}(0) = - i\sum\_{i=1}^nE[(X\_i)^3]$$
where $i=\sqrt{-1}$

I think that they applied the following formula: $E[(Y\_n)^3] = (-i)^3\phi^{(3)}\_{Y\_n}$. And then they move the "i" in the other side. But I'm not able to understand how. Because in my computation I have to multiply in both side for "i" at the power of something, therefore I never have $i^1$.