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Pi, the 16th Greek letter, lowercase π, uppercase Π, can refer to:

• pi (constant) - π ≈ 3.141592653589793238462643383279502884197169399375105820974944592307816406286208..., an irrational number representing the ratio of a circle's circumference to its diameter (also known as Archimedes' number)
• 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.
• Prime counting function π(x). (See also prime numbers.) A function defined to return a value equal to the quantity of prime numbers less than or equal to its inputted value. (eg $\pi(8) = 4$ since only four numbers: 2,3,5,7 are all the prime numbers less than or equal to 8.)
• The transformation (horizontal shift) of the Gamma function: $\Pi(x) = \Gamma(x+1)$.
• The reciprocal of its capitalized counterpart: $\pi(x) =\frac{1}{\Pi(x)}$.
• The upside down capital represent Coproducts: $\coprod_{j \in J}$