Psi (letter)

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 formula_1 is used as a variable for sets of sentences (in logic), and the lowercase formula_2 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.