Euler's totient function

Number of integers coprime to and less than n

In number theory, Euler's totient function counts the positive integers up to a given integer n {\displaystyle n} that are relatively prime to n {\displaystyle n} . It is written using the Greek letter phi as φ ( n) {\displaystyle \varphi (n)} or ϕ ( n) {\displaystyle \phi (n)} , and may also be called Euler's phi function. In other words, it is the number of integers k {\displaystyle k} in the range 1 ≤ k ≤ n {\displaystyle 1\leq k\leq n} for which the greatest common divisor gcd ( n , k) {\displaystyle \gcd(n,k)} is equal to 1.

From Wikipedia, under CC BY-SA. More on occurri.