pi (letter)

Pi (; Ancient Greek or , uppercase Π, lowercase π, cursive ϖ; ) is the sixteenth letter of the Greek alphabet, representing the voiceless bilabial plosive . 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 Latin P, Cyrillic Pe (П, п), Coptic pi (Ⲡ, ⲡ), and Gothic pairthra (𐍀).

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.
* In Mathematical finance, it represents a portfolio.

In science and engineering:
* The product operator in mathematics, indicated with capital pi notation (in analogy to the use of the capital Sigma as summation symbol).
* The osmotic pressure in chemistry.
* The viscous stress tensor in continuum mechanics and fluid dynamics.

Lowercase Pi

The lowercase letter 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, exactly half of the circle constant. The letter "" is the first letter of the Greek words 'periphery' and '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).
* Often inflation rate in macroeconomics.
* Sometimes profit in microeconomics.
* 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 (in the earliest polyamory pride flag design, created by Jim Evans in 1995, pi stands for the first letter of polyamory).

History

An early form of pi was , appearing almost like a gamma with a hook.

Variant pi

Variant pi or "pomega" ($\varpi\,\!$ or ϖ) is a glyph variant of lowercase pi sometimes used in technical contexts. It resembles a lowercase omega with a macron, though historically it is simply a cursive form of pi, with its legs bent inward to meet. It was also used in the minuscule script. It is a symbol for:
* Angular frequency of a wave in fluid dynamics (angular frequency is usually represented by $\omega$ but this may be confused with vorticity in a fluid dynamics context).
* Longitude of pericenter in celestial mechanics.
* Comoving distance in cosmology.
* Single-scattering albedo in radiative transfer.
* Mean fitness of a population in biology.
* Fundamental weights of a representation (probably to better distinguish from elements $w$ of the Weyl group, than the usual notation $\omega$).
* The lemniscate constant.

Unicode

Lower-case pi was fairly common in 8-bit character encodings, for instance it is at in CP437 and at on Mac OS Roman.
The various forms of pi present in Unicode are:

*
*
*
*
*
*
*
*
*
*

These are intended for use as mathematical symbols. Text written in the Greek language (i.e. words, as opposed to mathematics) should not come from any of the tables on this page, but instead should use the normal Greek letters, which have different code numbers and often a different appearance. Using the mathematical symbols to display words (or vice versa) is likely to result in inconsistent spacing and a clumsy, mismatched appearance:

*
*
*
*
*
*
*
*
*
*
*
*
*
*
*

See also

* П, п – Pe (Cyrillic)
* P, p – Pe (Latin)
* Greek letters used in mathematics, science, and engineering#Ππ (pi)
*Tau

References

{{Reflist

Greek letters