e is Euler's constant, the small number that stands at the heart of a wide range of mathematics. It's also the pivotal member of the most remarkable formula in math: the fundamental relation between e, from analysis, pi from trigonometry, and i from complex arithmetic. That it is even possible to find a common point among these far-flung regions of mathematics is remarkable. That it takes such concise and beautiful form is almost miraculous. Here are fifty pages that take on the task, as simply as possible, of getting to the essence of Euler's formula and the three worlds of math it unites.