Letter Psi: Twenty-third letter of the Greek alphabet
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 is used as a variable for sets of sentences (in logic), and the lowercase is used to represent the sum of reciprocals of Fibonacci numbers. Other applications include the polygamma function, the gamma function and the super golden ratio.
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