Well, title. Who chose the letter? I was thinking ancient Greeks (Euclidean? Pythagoras?) or was it chosen later on? Why did it stick and was not replaced like so many others?
In: 3
The earliest recorded use of the letter pi was from William Oughtred, an English mathematician from the 17th century. Pi denoted a circle’s “semi-perimeter” (a distance of one-half the circumference) and was used in concert with the Greek letter delta (for diameter) and rho (for radius) — Oughtred used delta and pi to denote the ratio of perimeter to diameter in his 1647 work *Clavis Mathematicae.*
Pi didn’t start to be used on its own to denote specifically the ratio of circumference to diameter until the 18th century, and its first use was by the Welsh scholar William Jones. He appears to have taken it from a contemporary by the name of John Machin, but no evidence survives of this.
It didn’t really take off until the renowned mathematician Leonhard Euler began using it in 1727; because Euler was regularly corresponding with mathematicians all across Europe, the use of the single letter pi rapidly took off from there.
Latest Answers