Any number can be raised to another number. Example 2 to the power of 3, designated here as 2 ** 3 is the same as 2 * 2 * 2. The number Pi is well known as an irrational number (no repeating digits) and is part of the ratio between the radius and the circumference of a circle.  The number e is Euler’s number and it is also an irrational number and is used in equations describing growth. The square root of -1, designated i is called an imaginary number since it cannot be described using any ‘real’ numbers, but it also has applications in our real world as it is used in equations describing electrical fields. It would seem that these two irrational numbers and the imaginary i could not have anything in common but in one of the most incredible formulas in math it has been proven that e ** (Pi * i) = -1. That is e raised to the power of (Pi times the square root of -1) equals -1. Mathematicians still do not know how to interpret this amazing correlation between three of the most fundamental numbers we know about.

(The Great Courses – Zero to Infinity: A History of Numbers)