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Psi (letter)
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For other uses, see Psi.
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 
Other letters  
Ϝϝ  Digamma  Ⱶⱶ  Heta 
Ϻϻ  San  Ϙϙ  Koppa 
Ϡϡ  Sampi  Ϛϛ  Stigma 
Ϸϸ  Sho  

Psi (uppercase/lowercase Ψ ψ) is the 23rd letter of the Greek alphabet. It is 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.
In mathematics, the uppercase [math]\Psi[/math] is used as a variable for sets of sentences (in logic), and the lowercase [math]\psi[/math] is used to represent the sum of reciprocals of Fibonacci numbers.^{[1]} Other applications include the polygamma function, the gamma function and the super golden ratio.^{[2]}