Xi

Xi (uppercase/lowercase Ξ ξ) is the 14th letter of the Greek alphabet. It is used to represent the "x" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 60. The Roman letter X and Cyrillic letter Ѯ came from Xi.

Greek alphabet
Αα Alpha Νν Nu Ββ Beta Ξξ Xi Γγ Gamma Οο Omicron Δδ Delta Ππ Pi Εε Epsilon Ρρ Rho Ζζ Zeta Σσς Sigma Ηη Eta Ττ Tau Θθ Theta Υυ Upsilon Ιι Iota Φφ Phi Κκ Kappa Χχ Chi Λλ Lambda Ψψ Psi Μμ Mu Ωω Omega
History
Archaic local variants Diacritics Ligatures Numerals ϛ (6) ϟ (90) ϡ (900)
Use in other languages
Bactrian Coptic Albanian
Related topics
Use as scientific symbols
Book Category Commons

In mathematics, both uppercase Ξ and lowercase ξ are used to refer to variants of Riemann Xi function (itself a variant of Riemann zeta function).[1] In physics, the uppercase Ξ is used to represent the cascade particle and xi baryon.[2]