Pi

Pi is the value expressing ratio of any circle's circumference to it's diameter.
This is endless number, and there is a person who memory the longest part of this and recorded to the Guinness book. I memory that he just memory the number all the day, and everyday.
I think Pi is one of the most beautiful number in the world, because of the Euler's formula. In this formula, e^(i Pi)=-1, natural logarithm raised by Pi times i is 1 negative.
I think this formula is the most beautiful one in the world of mathematics. This express the world of real number and imaginary number.