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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}