As I am introducing LaTeX to an audience, I'd like to show how it can render equations beautifully. As such, I'd like one equation (math/physics) that combines many different formatting requirements, such as integrals, sums, greek, other fonts, maybe even matrices.
This is (closely) secondary but it would be good if the equation had some theoretical significance. It would also be interesting if it involved matrices, but I don't think that's very possible.
Edit: For now, I have chosen the following equation, a functional equation that is crucial in the proof of the Prime Number Theorem:
If anyone has a better equation, please answer below! I'd love a better one.