Digamma

Digamma or Wau (uppercase/lowercase Ϝ ϝ ς) was an old letter of the Greek alphabet. It was used before the alphabet converted its classical standard form. It looked like a Latin "F", but it was pronounced like "w". In the 5th century BC, people stopped using it because they could no longer pronounce the sound "w" in Greek. However, they kept it as a sign for the number "6" in the system of Greek numerals. It was originally called "Wau" because of its sound. Later, when the sound was lost, it was called "Digamma", which means "double Gamma", because it looks like a Gamma (Γ) with two hooks. The Latin letter F was also taken over from Wau.
In mathematics, the name "digamma" is used in digamma function, which is the derivative of the logarithm of gamma function (that is, formula_1).