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Concept
Psi (Greek)
Summary
Psi '(p)saɪ,_"(p)si: (uppercase Ψ, lowercase ψ or ψ; ψι psi ˈpsi) is the twenty-third and penultimate letter of the Greek alphabet and is associated with a numeric value of 700. In both Classical and Modern Greek, the letter indicates the combination /ps/ (as in English word "lapse").
For Greek loanwords in Latin and modern languages with Latin alphabets, psi is usually transliterated as "ps".
The letter's origin is uncertain. It may or may not derive from the Phoenician alphabet. It appears in the 7th century BC, expressing /ps/ in the Eastern alphabets, but /kh/ in the Western alphabets (the sound expressed by Χ in the Eastern alphabets). In writing, the early letter appears in an angular shape ().
There were early graphical variants that omitted the stem ("chickenfoot-shaped psi" as: or ).
The Western letter (expressing /kh/, later /x/) was adopted into the Old Italic alphabets, and its shape is also continued into the Algiz rune of the Elder Futhark.
Psi, or its Arcadian variant or was adopted in the Latin alphabet in the form of "Antisigma" (Ↄ, ↃC, or 𐌟) during the reign of Emperor Claudius as one of the three Claudian letters. However, it was abandoned after his death.
The classical Greek letter was adopted into the early Cyrillic alphabet as "Ѱ".
The letter psi is commonly used in physics to represent wave functions in quantum mechanics, such as in the Schrödinger equation and bra–ket notation: . It is also used to represent the (generalized) positional states of a qubit in a quantum computer.
Psi is also used as the symbol for the polygamma function, defined by
where is the gamma function.
The letters Ψ or ψ can also be a symbol for:
Psychology, psychiatry, and sometimes parapsychology (involving paranormal or relating with the supernatural subjects, especially research into extrasensory perception).
In mathematics, the reciprocal Fibonacci constant, the division polynomials, and the supergolden ratio.
Water potential in movement of water between plant cells.
In biochemistry, it denotes pseudouridine, an uncommon nucleoside.
Official source
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