Riemann zeta function, Riemann hypothesis, Theory of higher dimensions, Riemannian metric

The Riemann Zeta Function: http://mathworld.wolfram.com/RiemannHypothesis.html

Because its mathmatics greatest unsolved problem.

It is not so much an equation, as a function. Since the definition is fairly complicated, I suggest you visit the Wikipedia page: http://en.wikipedia.org/wiki/Riemann_zeta_function

... or do additional Web searches either for "Riemann hypothesis" or "Riemann zeta function".

it is used in complex function and used for studying the Riemann zeta-function along the important line where the real part of the argument is one-half.