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?

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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 $e=mc^2$ 

