Euler's totient function
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.