 Euler introduced much of the mathematical notation in use today, such as the notation "f"("x") to describe a function and the modern notation for the trigonometric functions.He was the first to use the letter "e" for the base of the natural logarithm, now also known as Euler's number.The use of the Greek letter formula_1 to denote the ratio of a circle's circumference to its diameter was also popularized by Euler (although it did not originate with him).