In this formula for Euler numbers:
\begin{equation*} A_n = i^{n+1}\sum _{k=1}^{n+1} \sum _{j=0}^k{k\choose{j}} \frac{(-1)^j(k-2j)^{n+1}}{2^ki^kk}, \end{equation*}
what is $i$? I have to generate the $n^{th}$ Euler number, and I don't have $i$.
It is not the imaginary unit, is it?