Certain formulas I really enjoy looking at like the EulerMaclaurin formula or the Leibniz integral rule. What's your favorite equation, formula, identity or inequality?
closed as no longer relevant by Robin Chapman, Akhil Mathew, Yemon Choi, Qiaochu Yuan, Pete L. Clark Aug 22 '10 at 9:00This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question. 


With the stuff I've seen in the literature of sequence transformations, I've started to love the formulae for Aitken's Δ² process: $S_n^{\prime}=S_{n+1}\frac{(\Delta S_n)^2}{\Delta^2 S_n}$ and its generalization the Wynn ε algorithm: $\varepsilon_{k+1}^{(n)}=\varepsilon_{k1}^{(n+1)}+\frac1{\varepsilon_{k}^{(n+1)}\varepsilon_{k}^{(n)}}$ for the latter one especially because it is nicely represented as a lozenge diagram: 


I like RiemannRoch the most!!! 


I'm surprised that nobody said $e=mc^2$ 

