# Proving that $f(n)=n$ if $f(n+1)>f(f(n))$

How can we prove that if $f:\mathbb{N}\rightarrow\mathbb{N}$ is a function so that $f(n+1)>f(f(n))$ for all $n\in\mathbb{N}$ then $f(n)=n$ for all $n\in\mathbb{N}$?

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Where did you get this problem? – Will Jagy Dec 2 '12 at 4:16
a book Mathematical Olympiad – Roiner Segura Cubero Dec 2 '12 at 4:29
Fine. Authors, full title, year published, page number please? – Will Jagy Dec 2 '12 at 4:32
– Roiner Segura Cubero Dec 2 '12 at 4:34
IMO 1977 problem 6. My g/f found it online:) – Tengu Dec 2 '12 at 15:05

Claim 1: If $f(k) = 0$ then $k =0$.

Proof: Suppose not. Then there exists $k$ such that $0 = f(k) > f(f(k-1))$, which is not possible, as $f: \mathbb{N} \mapsto \mathbb{N}$.

Claim 2: $f(0) = 0$.

Proof: Let $S = \{f(k) | k > 0\}$. Let $a$ be the smallest number in $S$. Then there exists $k >0$ such that $a = f(k) > f(f(k-1))$. But this means $f(k-1) = 0$. Thus $k=1$, and $f(0) = 0$.

Claim 3: $f(n) = n$.

Proof: Assume, for all $0 \leq m < n$, that $f(k) = m$ iff $k = m$. Now we proceed as in the proofs of Claims 1 and 2.

If $f(k) = n$, then $f(f(k-1)) < n$, which means $k-1 = f(k-1) = f(f(k-1)) < n$. So $k < n+1$, which means that $k = n$.

Let $S = \{f(k) | k > n\}$. Let $a$ be the smallest number in $S$. Thus there exists $k >n$ such that $a = f(k) > f(f(k-1))$. Therefore $f(k-1) \leq n$. But if $f(k-1) < n$, then $f(k-1) = k-1$, and so $k \leq n$. But $k>n$. Therefore, $f(k-1) = n$, and so $k= n+1$ and $f(n) = n$.

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Mike. I do not understand your test sequence. In the penultimate sentence why if $f (k-1) <n$ then $f (k-1) = k-1$? – Roiner Segura Cubero Dec 2 '12 at 18:35
@RoinerSeguraCubero: If $f(k-1)<n$ then the induction assumption "for all $0 \leq m < n$, $f(k)=m$ iff $k=m$ " applies. Thus $f(k-1)=k-1$. – Mike Spivey Dec 2 '12 at 21:03
Mike, if $f(k-1)<n$ and if we apply the induction hypothesis then we have $f(f(k-1))=f(k-1)$ and only if f is injective we can say that $f(k-1)=k-1$. Or am I wrong? – Roiner Segura Cubero Dec 2 '12 at 21:44
@RoinerSeguraCubero: No, I'm not making any global assumptions about $f$ being injective. The induction hypothesis is that if $f$ maps $k$ to any value $m$ smaller than $n$ then $k$ must be $m$, and so $f$ maps $k$ to itself. I suppose the induction hypothesis could be thought of implying that $f$ is injective in a local sense, but I didn't think of it that way. In other words, $f(k-1) = k-1$ precisely because $f(k-1) < n$. If $f(k-1) \geq n$, then we wouldn't be able to claim that $f(k-1) = k-1$ (at least at this point in the proof.) – Mike Spivey Dec 2 '12 at 21:47
Excellent Mike. Thanks :) – Roiner Segura Cubero Dec 3 '12 at 0:49

Attempted solution (Improvements/corrections are greatly welcome!):

Clearly, the function isn't constant on any interval. So it must be either strictly increasing or strictly decreasing.

Suppose the function isn't injective. Then there exist $a,b \in \mathbb{N}$ such that $f(a) = f(b)$, but $a \neq b$. So either $a>b$ or $a<b$. WLOG, let $a>b$. Then $a = b+m$, for some $m \in \mathbb{N}$. Now, $f(a) = f(b+m) > f(f(b+(m-1)) > f(f(f(b+m-2) > ... >f(f(...(f(b))...)$. Now, suppose, WLOG that $f$ is increasing (if $f$ is decreasing, then there is also a contradiction). Then $f(f(...(f(b))...) > f(b)$. But $f(b) = f(a)$. Thus, we have a contradiction. So, $f$ must be injective.

Now, apply $f^{-1}$ on both sides, and you get that $n+1 > f(n)$, $\forall n \in \mathbb{N}$.

Clearly, if $\forall n \in \mathbb{N}$, $f(n) = a*n+b$, where $a \neq 1$ and $b \neq 0$, then it would contradict $n+1 > f(n)$, $\forall n \in \mathbb{N}$.

Now it's just left to see what happens if there exists an $n'$ where $f(n') \neq n'$. In such a case, $f(n')>n'$ or $f(n')<n'$. Check that both cases still lead to contradictions.

I think if you can somehow rule out that $f$ is decreasing, then this might work.

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Hello, thanks for the advice. I think I've completed my answer for this question, and will be sure to follow your advice in the future. – chubbycantorset Dec 2 '12 at 3:18
Cubby why $f$ is increase? – Roiner Segura Cubero Dec 2 '12 at 3:29
You are right, it doesn't have to be increasing. Let me improve my solution. – chubbycantorset Dec 2 '12 at 3:38
Ah, now I remember why it has to be strictly increasing. Because if it is strictly decreasing, then $f(0) < 0$, which implies that $f(0) \notin \mathbb{N}$. – chubbycantorset Dec 2 '12 at 3:46
Not constant on any interval is not the same as strictly increasing or decreasing. The sequence that alternates 0,1 is not constant nor strictly increasing or decreasing. – Ross Millikan Dec 2 '12 at 4:15