Xi (letter)
Xi is the fourteenth letter of the Greek alphabet, representing the voiceless consonant cluster. Its name is pronounced 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 Image:Phoenician [samekh.svg|20px].
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.Because this variant of the Greek alphabet was used in Magna Graecia, 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. 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, equal to 1018 Wei
Lowercase
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
- The radial integral in the spin-orbit matrix operator in atomic physics.
- The Killing vector in general relativity.
- Average logarithmic energy decrement per collision
- 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 and Hb-Gower I
- 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