In all, Euler's legacy included the introduction of the concept of functions, as well as the proper way to write them within a mathematical formula by using the notation f(x). Before his formalization of the notation, functional relationships were referred to by just one letter, F, or with Greek letters omitting the parentheses, φx. In fact, Euler spelled out much of the mathematical notation we use today, including the letter "e" for the base of the natural logarithm (also known as Euler's number), the letter "I" to denote the imaginary unit, and the Greek letter "Σ" for summations. He also encouraged the use of the Greek letter "π" to signify the ratio of a circle's circumference to its diameter.

Which mathematical concepts did Euler's legacy include? Check all that apply.