Pi (letter)
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Pi (uppercase Π, lowercase π and ϖ; Template:Lang-el Template:IPA-el) is the sixteenth letter of the Greek alphabet, representing the sound Template:IPA. In the system of Greek numerals it has a value of 80. It was derived from the Phoenician letter Pe (16px). Letters that arose from pi include Latin P, Cyrillic Pe (П, п), Coptic pi (Ⲡ, ⲡ), and Gothic pairthra (𐍀).
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Uppercase Pi
The uppercase letter Π is used as a symbol for:
- 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 product operator in mathematics, indicated with capital pi notation Template:Math (in analogy to the use of the capital Sigma Template:Math as summation symbol).
- The osmotic pressure in chemistry.
- The viscous stress tensor in continuum mechanics and fluid dynamics.
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Lowercase Pi
The lowercase letter π is used as a symbol 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 Template:Wikt-lang 'periphery' and Template:Wikt-lang '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.
[[File:Polyamory Pride Flag.svg|thumb|right|The earliest polyamory pride flag design, created by Jim Evans in 1995, in which the lowercase letter π stands for the first letter of polyamory.]]
- Polyamory (in the earliest polyamory pride flag design, created by Jim Evans in 1995, pi stands for the first letter of polyamory).
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