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i.e. Find the least integer $B$ such that $4B$ is not in the image of $\phi$. The original problem is to find a $B$ such that $4B$ is not an order for some multiplicative group $\mathbb{Z}^*_{n}$. The answer is $B=17$.

I had read the two reference below.

How to solve the equation $\phi(n) = k$?

Some remarks on Euler’s totient function

But I do not know where to start to tackle this problem.

I suppose it may related to the fact that

if $\gcd(m, n) = 1$, then $\phi(mn) = \phi(m) \phi(n)$.

So we may start with $\phi(5)=4$
But at second thought, if $gcd(m, n) \neq 1$, the there is factor as $gcd(m,n)\over \phi( gcd(m,n) )$, which can had factor of 4. My qeustion is, besides try and error, is there other way to find $B$ by hand?

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  • $\begingroup$ The even nontotients are given in oeis.org/A005277. $68$ is the first one divisible by $4$. From the comments "If p is prime then the following two statements are true. I. 2p is in the sequence iff 2p+1 is composite (p is not a Sophie Germain prime). II. 4p is in the sequence iff 2p+1 and 4p+1 are composite. - Farideh Firoozbakht, Dec 30 2005" You can verify that $68$ satisfies II $\endgroup$ – Ross Millikan May 1 '17 at 19:42
  • $\begingroup$ Thank alot. The "4p is in the sequence iff 2p+1 and 4p+1 are composite" apply to the conidtion B is prime. How abot the case B is not a prime? $\endgroup$ – Rikeijin May 1 '17 at 19:51
  • $\begingroup$ Then it takes more work. You use the prime factorization of B and reflect on the link between the prime factorization of B and $\phi(B)$ and the multiplicative nature of $\phi$. $\endgroup$ – Ross Millikan May 1 '17 at 19:54
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First prove that $B=17$ satisfies the requirement, so there is no $n$ with $\phi(n)=68$. As $\phi$ is multiplicative, to get a factor $17$ in $\phi(n)$ you either need $17^2$ dividing $n$, but then you get four factors of $2$ in $\phi(n)$, or a prime of the form $2\cdot 17+1$ or $4 \cdot 17 +1$, but those are not prime. Any higher prime of the form $17k+1$ will give more factors besides $17$ than $4$. Now just show that all smaller B fail by displaying an $n$ with the proper $\phi(n)$. $\phi(5)=4, \phi(16)=8,$ etc. They are not hard to find for small numbers if you think about the link between prime factorization and totient.

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