Number Theory :

Prove for all $m \in \mathbb{N}$ exists $n \in \mathbb{N}$ such that :

$\varphi(n)-\varphi(n+1)>m$ and $\varphi(n)-\varphi(n-1)>m$


For $m\in \mathbb{N}$ let $q_m$ be prime number form of $4k+3$ for some $k\in \mathbb{N}$ such that $2m+3<q_m$

  • $\begingroup$ Get some ideas from WA. $\endgroup$
    – Neat Math
    Dec 17, 2020 at 22:40
  • $\begingroup$ @NeatMath Thanks , I will check it out . maybe it will give me a ways to solve $\endgroup$
    – ATB
    Dec 17, 2020 at 22:43

1 Answer 1


Hint: Suppose $n$ is an odd prime. Then $\varphi(n) = n-1$. Also, $n-1$ and $n+1$ are both even, and thus, $\varphi(n-1) \le \dfrac{n-1}{2}$ and $\varphi(n+1) \le \dfrac{n+1}{2}$. Do you see why this is true?

Using these results, how large does $n$ need to be to guarantee that $\varphi(n)-\varphi(n-1) > m$ and $\varphi(n)-\varphi(n+1) > m$?

  • $\begingroup$ First of all Thanks you for that Hint, I am understand the hint and I can see that always true because every even number in $\varphi$ great or equal then even number divide by two. For the second line I think that n need to be large as more then m divide by two. I don't sure about that . $\endgroup$
    – ATB
    Dec 20, 2020 at 22:10
  • $\begingroup$ Using $\varphi(n) = n-1$ and $\varphi(n+1) \le \tfrac{n+1}{2}$, can you find a lower bound for $\varphi(n)-\varphi(n+1)$? Then, set that lower bound $> m$ and solve for $n$. $\endgroup$
    – JimmyK4542
    Dec 20, 2020 at 22:13
  • $\begingroup$ Hey, I've been stuck for a long time with this proof. Can I have your help please ? $\endgroup$
    – ATB
    Dec 30, 2020 at 20:36
  • $\begingroup$ We want to pick an odd prime $n$ such that both $\varphi(n)-\varphi(n+1) \ge (n-1)-\tfrac{n+1}{2} = \tfrac{n-3}{2} > m$ and $\varphi(n)-\varphi(n-1) \ge (n-1)-\tfrac{n-1}{2} = \tfrac{n-1}{2} > m$ hold. Can you use these inequalities to figure out how big $n$ needs to be? Then the proof goes something like "For any $m \in \mathbb{N}$, pick an odd prime $n > ?$. Then $\varphi(n)-\varphi(n+1) \ge \ldots > m$ and $\varphi(n)-\varphi(n-1) \ge \ldots > m$." $\endgroup$
    – JimmyK4542
    Dec 30, 2020 at 21:31

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