Xi (letter)

Xi (/zaɪ/ ⓘ ZY or /(k)saɪ/ (K)SY; uppercase Ξ, lowercase ξ; Greek: ξι) is the fourteenth letter of the Greek alphabet, representing the voiceless consonant cluster [ks]. Its name is pronounced [ksi] ⓘ in both Ancient Greek and Modern Greek. In the system of Greek numerals, it has a value of 60. Xi was derived from the Phoenician letter samekh (𐤎).

Xi is distinct from the letter chi, which gave its form to the Latin letter X.

Greek

Both in classical Ancient Greek and in Modern Greek, the letter Ξ represents the consonant cluster /ks/. In some archaic local variants of the Greek alphabet, this letter was missing. Instead, especially in the dialects of most of the Greek mainland and Euboea, the cluster /ks/ was represented by Χ (which in classical Greek is chi, used for /kʰ/).

Because this variant of the Greek alphabet was used in Magna Graecia (the Greek colonies in Sicily and the southern part of the Italian peninsula), the Latin alphabet borrowed Χ rather than Ξ as the Latin letter that represented the /ks/ cluster that was also present in Latin.

Cyrillic

Xi was adopted into the early Cyrillic alphabet as the letter ksi (Ѯ ѯ). It was ultimately removed from the Russian alphabet in 1735.

Mathematics and science

Uppercase

The uppercase letter Ξ is used as a symbol in various contexts.

Pure mathematics

Harish-Chandra's Ξ function in harmonic analysis and representation theory
The Riemann Xi function in analytic number theory and complex analysis

Physics

The "cascade particles" in particle physics
The partition function under the grand canonical ensemble in statistical mechanics

Other uses

Indicating "no change of state" in Z notation in computing
Used as the Latin letter E in some logos.
Monetary units of the cryptocurrencies Ether (and less commonly ETC), equal to 1018 Wei

Lowercase

The lowercase letter ξ is used as a symbol for:

Pure mathematics

Random variables
A parameter in a generalized Pareto distribution
The symmetric function equation of the Riemann zeta function in mathematics, also known as the Riemann xi function
A universal set in set theory
A number used in the remainder term of Taylor's theorem that falls between the limits a and b
A number used in error approximations for formulas that are applications of Taylor's theorem, such as Newton–Cotes formulas

Physics and astronomy

In fluid dynamics, the Iribarren parameter.
The initial mass function in astronomy.
The correlation function in astronomy.
Spatial frequency; also sometimes temporal frequency.
A small displacement in MHD plasma stability theory
The x-coordinate of computational space as used in computational fluid dynamics
Potential difference in physics (in volts)
The radial integral in the spin-orbit matrix operator in atomic physics.
The Killing vector in general relativity.
Average logarithmic energy decrement per collision (neutron calculations in nuclear physics)
Pippard's cohesion length in superconductors
The diameter of a crystal nucleus in nucleation theory
Microturbulence velocity in a stellar atmosphere
The dimensionless longitudinal momentum loss of a beam particle after a two-body interaction in accelerator physics.
Dimensionless distance variable used in the Lane–Emden equation

Other uses

Propositional variables in some philosophical works, first found in Wittgenstein's Tractatus
Extent of reaction, a concept in physical chemistry used often in chemical engineering kinetics and thermochemistry
Unknown stereochemistry or stereocentre configuration in a planar ring system in organic chemistry, as well as uppercase Xi for unknown R/S/E/Z configuration in general
One of the two different polypeptide chains of the human embryonic hemoglobin types Hb-Portland (ξ2γ2) and Hb-Gower I (ξ2ε2)
A parameter denoted as warped time used to derive the equations for homogeneous azeotropic distillation
State Price Density in mathematical finance
The information vector in the Information Filter, GraphSLAM, and a number of other algorithms used for robot localization and robotic mapping.
Used in Support Vector machines in cases where the data is not linearly separable.
Used in Microelectronics to represent the distance from a p-n junction to a point in the depletion region where the electric field is strongest.

Other uses

Uppercase Ξ is used as an 'E' to stylise company names/logos like Razer (styled as RΛZΞR), Tesla (styled as TΞSLA), the presidential campaigns of Joe Biden (styled as BIDΞN), musician Banners (styled as BANNΞRS), and in South Korean boy group ZE:A's newest logo (styled as ZΞA) making a faux greek (Compare: Heavy Metal umlaut; Faux Cyrillic)

Unicode

Unicode Code Charts:

U+039EΞ GREEK CAPITAL LETTER XI (Ξ) (\Xi in TeX)
U+03BEξ GREEK SMALL LETTER XI (ξ) (\xi in TeX)
U+2C9CⲜ COPTIC CAPITAL LETTER KSI
U+2C9Dⲝ COPTIC SMALL LETTER KSI
U+1D6B5𝚵 MATHEMATICAL BOLD CAPITAL XI
U+1D6CF𝛏 MATHEMATICAL BOLD SMALL XI
U+1D6EF𝛯 MATHEMATICAL ITALIC CAPITAL XI
U+1D709𝜉 MATHEMATICAL ITALIC SMALL XI
U+1D729𝜩 MATHEMATICAL BOLD ITALIC CAPITAL XI
U+1D743𝝃 MATHEMATICAL BOLD ITALIC SMALL XI
U+1D763𝝣 MATHEMATICAL SANS-SERIF BOLD CAPITAL XI
U+1D77D𝝽 MATHEMATICAL SANS-SERIF BOLD SMALL XI
U+1D79D𝞝 MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL XI
U+1D7B7𝞷 MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL XI

External links

Media related to Xi (letter) at Wikimedia Commons