Guess the Equation: Euler's Identity

[eprjenkins]

Just like the maths question and answer game; except with guessing the names of famous equations/theorems etc.

Someone posts an equation and another identifies it. They then post their own equation.

Here is the first, an easy one:

$$e^{i\pi} + 1 = 0$$

 

[arildno]

Archimedes' non-equality.

Here's mine:
2+2=4

 

[eprjenkins]

I got owned.