\begin{equation}
E(q) = E^{[0]} + E^{[1]}q + \frac{1}{2}E^{[2]}q^2 + \frac{1}{6}E^{[3]}q^3 + \ldots;\quad
       E^{[n]} = \left.\frac{\partial^n E}{\partial q^n}\right|_{q=0}
\end{equation}