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Pi (Π, π) is the sixteenth letter of the Greek alphabet, representing the sound [p]. In the system of Greek numerals it has a value of 80. It was derived from the Phoenician letter Pe. Letters that arose from pi include Cyrillic Pe, Coptic pi, and Gothic pairthra.

The uppercase Pi (Π) is used for:

The product operator in mathematics, indicated with capital pi notation Π (in analogy to the use of the capital Sigma Σ as summation symbol).

The osmotic pressure in chemistry.

The viscous stress tensor in continuum mechanics and fluid dynamics.

Other:

In textual criticism, Codex Petropolitanus, a 9th-century uncial codex of the Gospels, now located in St. Petersburg, Russia.

In legal shorthand, it represents a plaintiff.

The lowercase Pi (π) is used for:

The mathematical real transcendental (and thus irrational) constant π = 3.14159..., the ratio of a circle's circumference to its diameter in Euclidean geometry. The letter π is the first letter of the Greek words περιφέρεια periphery and περίμετρος perimeter, i.e. the circumference.

The prime-counting function in mathematics.

Homotopy groups in algebraic topology.

Dimensionless parameters constructed using the Buckingham π theorem of dimensional analysis.

The hadron called the pion (pi meson).

Economic profit in microeconomics.

Inflation rate in macroeconomics.

A type of chemical bond in which the p orbitals overlap, called a pi bond.

The natural projection on the tangent bundle on a manifold.

The unary operation of projection in relational algebra.

Policy in reinforcement learning.

Polyamory.

The letters of the Ancient Greek Alphabet, which are frequently utilized in math and science:

“Pi (Letter).” Wikipedia, Wikimedia Foundation, 17 Apr. 2020, en.wikipedia.org/wiki/Pi_(letter).

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