Properties of Euler Totient Function

Properties of Euler Totient Function:

• $\varphi (1)=1$
• $\varphi (p)=p-1$ if $p$ is prime
• $\varphi (p\times q)=\varphi (p)\times \varphi (q)=(p-1)\times (q-1)$ if $p$ and $q$ are prime and $p\ne q$