# Psi (letter): Wikis

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# Encyclopedia

 Greek Obsolete letters Other characters 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 Ϝϝ Digamma Ϙϙ Qoppa Ϻϻ San Ϡϡ Sampi Ϛϛ Stigma Ϸϸ Sho Ͱͱ Heta Greek diacritics
For other uses, see Psi.

Psi (uppercase Ψ, lowercase ψ; pronounced in English as sigh) is the 23rd letter of the Greek alphabet and has a numeric value of 700. In both Classical and Modern Greek, the letter indicates the combination /ps/ (like in English "lapse"). The letter was adopted into the Old Italic alphabet, and its shape is continued into the Algiz rune of the Elder Futhark. Psi was also adopted into the early Cyrillic alphabet as Ѱ. In Greek loanwords in Latin and modern languages with Latin alphabets, Psi is usually transliterated as "ps". In English, due to phonotactic constraints, its pronunciation is usually simplified to /s/.

The letter psi is commonly used in physics for representing a wavefunction in quantum mechanics, particularly with the Schrödinger equation and bra-ket notation: $\langle\phi|\psi\rangle$. 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

$\psi_n(x) = \frac{d^{(n)}}{dx^{(n)}}\frac{\Gamma '(x)}{\Gamma (x)}\,\!$

where Γ(x) is the gamma function.

The letters Ψ or ψ can also be a symbol for:

# Simple English

 Common letters Uncommon letters Other letters Αa A Μm Em Βb Be Νn En Gg Ge Οο O Δd De Πp Pe h Ha Жж Zhe Εɛ E Rr Ar Ζz Ze Ss Es Ηe Ee Τt Te Θө Eth Uu U Ιi I Ff Ef Jj Je Ww Wa Κk Ka Χx Xa Ll El ОWоw We Vv Ve Digamma Qoppa San Sampi Stigma Sho Greek alphabet

Psi (uppercase/lowercase Ψ ψ), is the letter of the Greek alphabet, used to represent the "ps" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 700. Letters that came from it include Cyrillic Ѱ.