# Prove that there are infinitely many even nontotients

How do I show that there are infinitely many even numbers which are nontotients ?

I have proved that $\phi(n)$ is even $\forall\space n>2$ by showing that for the set $Z_n^*$ of all natural numbers less that and coprime to $n,\space |Z_n^*|$ is even.

This I did by showing that $1\in Z_n^*, \space n-1 \in Z_n^*$ and $\forall x \in Z_n^*, \space x^{-1} mod \space n \in Z_n^*$. Also $\forall x \ne {1,n-1} \space x\ne x^{-1}$.

I am unable to think of a way to prove that there are infinitely many even numbers k such that $\forall n \space \phi(n) \ne k$

• Here are some infinite subsets of nontotients: oeis.org/A005277 Maybe you could start with these. – Dzoooks Aug 15 '18 at 18:07

Suppose, $N=2p$ where $p\ge 5$ is prime and $2p+1$ is not prime. To ensure that $2p+1$ is not prime, it is enough to choose $p\equiv 1\mod 3$
If $\varphi(M)=N$ , then for every prime factor $q\mid M$ we have $q-1\mid N$
Hence , only the prime factors $2$ and $3$ are possible because neither $p+1$ nor $2p+1$ is prime.
But if $M$ only has prime factors $2$ and $3$, $\varphi(M)$ must be a power of $2$ contradicting $\varphi(M)=2p$. Hence , $N$ is a non-totient.