The Second Most Beautiful Equation in the World of Mathematics

Euler. Source: Wikimedia Commons and self-editing.

I already wrote an article on Euler’s identity equation, arguably the most beautiful equation in the world. This article is about Euler's characteristic. Whenever I start writing an article on mathematics, my mind tells me to put ‘This article is sponsored by Leonhard Euler’ in the subtitle. There’s probably not even a single field in mathematics in which Euler didn’t contribute…