@Jun-GooKwak Use \in for belonging and \varepsilon for epsilon. Alternatively, use \epsilon for belonging (crescent moon looking) and use the \varepsilon (wiggly) for epsilon.
Compare $\Large\epsilon$ (crescent) and $\Large\varepsilon$ (wiggly).
@r9m: One of my favorite uses of the Euler-Maclaurin Sum Formula is to compute $\displaystyle\sum_{k=1}^\infty(-1)^k\frac{\log(k)}{k}$. I just used that in an answer :-)
