**Pi - Wikipedia**
https://en.wikipedia.org/wiki/Pi

The symbol used by mathematicians to represent the ratio of a circle's circumference to its diameter is the lowercase Greek letter π, sometimes spelled out as pi, and derived from the first letter of the Greek word perimetros, meaning circumference. In English, π is pronounced as "pie" (/paɪ/ PY). In mathematical use, the lowercase letter π is distinguished from its capitalized and enlarged counterpart Π, which denotes a product of a sequence, analogous to how Σ denotes summation. The choice of the symbol π is discussed in the section Adoption of the symbol π.

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