Equation (A.4) is known as Euler’s formula, after Swiss polymath Leonhard Euler. When we set θ = π, we get eiπ = cos π + i sin π = -1 + 0, which is equivalent to the famous expression (A.5) There’s a good case to be made that 0, 1, i, π, and e are the five most important numbers in all of mathematics. This formula relates them in a stylish way.
