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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?

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    Note that the algorithm in the paper mentioned by @Moron can be implemented with just a one-dimensional array.2010-10-28

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Yes, it is $\sqrt{-1}$. They will all divide out in the end. A derivation of your formula is here. The $i$'s come from changing the $\sin(x)$ function to its definition in terms of $\exp(x)$

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    A lot of references are found on OEIS: http://www.research.att.com/~njas/sequences/?q=A000364&sort=0&fmt=0&language=english&go=Search2010-10-27