Xi (letter)

Xi (uppercase Ξ, lowercase ξ) is the 14th letter of the Greek alphabet. It is IPA: [ksi] in Modern Greek, and generally Template:PronEng (UK) or /ˈzaɪ/ (US) in English. In the system of Greek numerals, it has a value of 60. Its Latin equivalent is X.

The lower-case letter ξ is used as a symbol for

A joined variant of Ξ.

• random variables;
• extent of reaction (a topic found most often in chemical engineering kinetics);
• eigenvectors;
• 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;
• the damping factor in an RLC circuit;
• a universal set in the set theory;
• a number used in the error term of Newton-Cotes formulas that falls between a and b;
• one of the two different polypeptide chains of the human embryonic hemoglobin types Hb-Portland (ξ₂γ₂) and Hb-Gower I (ξ₂ε₂);
• the correlation function in astronomy;
• frequency.

The upper-case letter Ξ is used as symbol for

• The 'cascade particles' in particle physics;
• indicating "no change of state" in Z notation.

Xi was also adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ).