History, Terminology, and Notation
Leonhard Euler introduced the function in 1760. The standard notation φ(n) is from Gauss' 1801 treatise Disquisitiones Arithmeticae. Thus it is usually called Euler's phi function or simply the phi function.
In 1883 J. J. Sylvester coined the term totient for this function, so it is also referred to as the totient function, the Euler totient, or Euler's totient. Jordan's totient is a generalization of Euler's.
The cototient of n is defined as n – φ(n), i.e., the number of positive integers less than or equal to n that are divisible by at least one prime that also divides n.
Read more about this topic: Euler's Totient Function