Xi (letter)
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Xi (uppercase Ξ, lowercase ξ) is the 14th letter of the Greek alphabet. It is generally pronounced "squiggle" in English, following the Greek pronunciation, although the pronunciation [[IPA|[zaɪ]]] is also used by some speakers of American 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
- 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;
- one of the two different polypeptide chains of the human embryonic hemoglobin types Hb-Portland (ξ2γ2) and Hb-Gower I (ξ2ε2).
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).