Pi

Pi, the 16^th Greek letter, written in lowercase as π and capitalized as Π (sometimes written $\mathrm {][}$), can refer to the following mathematical concepts.

• The mathematical constant $\pi \approx 3.14159\ldots$, representing the ratio of a circle's circumference to its diameter (also known as Archimedes's number)
• The product operator Π — e.g., $\prod _{i=0}^{n}f(i)$. (See also product of a sequence.) It is functionally similar to summation notation, using $\sum _{i=0}^{n}$, which is the sum of a sequence.
• The prime counting function π(x). (See also prime numbers.)
• The transformation (horizontal shift) of the Gamma function: $\Pi(x)=\Gamma(x+1)$.
  • The reciprocal of its capitalized counterpart: $\pi (x)={\frac {1}{\Pi (x)}}$.