Euler is credited for popularizing the Greek letter π (lowercase pi) to denote the ratio of a circle’s circumference to its diameter, as well as first using the notation f(x) for the value of a function, the letter i to express the imaginary unit √-1, the Greek letter Σ (capital sigma) to express summations, the Greek letter Δ (capital delta) for finite differences, and lowercase letters to represent the sides of a triangle while representing the angles as capital letters.