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Show that there are no functions $f: \mathbb R \to \mathbb R$ which have the Darboux property (the intermediate value property) and $f(f(x))=\cos^2(x) ; \ \forall \ x\in \mathbb R$.

I guess that I'd have to use the fact that $f(f(x)) \in [0,1]$, but I'm not sure how

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If you google "f(f(x)) math overflow" you might get some hints. –  NikolajK Mar 30 '13 at 11:50
Ok I give up >_<. Here are the few things I found in case it rings a bell: $\forall \ x\in \Bbb R,f(f(x))=\cos^2(x)\space\space\space\space\space\space\space\space\space\space$ $\forall x \in \Bbb R, f(f(x))\in [0,1]\space\space\space\space\space\space\space\space\space\space$ $\forall x \in \left[0,\cfrac{\pi}{2}\right], f(f(x))=\cos^2(x) \Leftrightarrow \cos(x)=\sqrt{f(f(x))} \Leftrightarrow x=\arccos \left(\sqrt{f(f(x))}\right)\space\space\space\space\space\space\space\space$ So $f$ is injective on $\left[0,\cfrac{\pi}{2}\right]$ –  xavierm02 Mar 30 '13 at 12:50
$\forall x \in \Bbb R, \cos^2(f(x))=f(f(f(x)))=f(\cos^2(x))\space\space\space\space\space$ $f(1)=\cos^2(f(0))\space\space\space\space\space$ $f(0)=\cos^2\left(f\left(\cfrac{\pi}{2}\right)\right)$ –  xavierm02 Mar 30 '13 at 12:51
did you find out how in the meantime? i completed my answer, let me know your thoughts –  suissidle Apr 6 '13 at 12:46

1 Answer 1

up vote 4 down vote accepted

ok so i took this with me on easter and found a proof. i'll post more details later, first only a few hints to get you started. as you already found out, $f|_{[0,\frac{\pi}{2}]}$ is injective; actually, for each $n\in\mathbb{Z}$, $f|_{[n\frac{\pi}{2},(n+1)\frac{\pi}{2}]}$ is injective. using the IVP, show that each $f|_{[n\frac{\pi}{2},(n+1)\frac{\pi}{2}]}$ is continuous, hence $f$ must be continuous everywhere. show that without loss of generality we can assume $f\ge0$ and $f(-x)=f(x)$. show $f$ is periodic with period $\pi$ (so $f$ attains its maximum and minimum at the endpoints of each $[n\frac{\pi}{2},(n+1)\frac{\pi}{2}]$) and a few more things to derive a contradiction using $x=0$ and $x=\frac{\pi}{2}$. you may or may not need the fact that $f$ has exactly one fixed point $x^*$, and $x^*\in(0,1)$, and $x^*$ is also a fixed point of $\cos^2(x)$

EDIT A solution is a function that satisfies andreea's problem (we want to prove that no such exists). Let $f$ denote any solution, unless stated otherwise. IVP stands for intermediate value property, and monotone refers to either monotonically increasing or monotonically decreasing.

Lemma 1 Let $J$ be any set, $I\subseteq J$ any subset, and $f:J\rightarrow J$ any function. Then

$$(f\circ f)|_I \quad \text{is injective} \quad \Longrightarrow \quad f|_I \quad \text{is injective.}$$

Proof Suppose $(f\circ f)|_I$ is injective, $x,y\in I$, and $f(x)=f(y)$. Then $f(f(x))=f(f(y))$, hence $x=y$.

Take $J=\mathbb{R}$, $n\in\mathbb{Z}$, $I=[n\frac{\pi}2,(n+1)\frac{\pi}2]$. Since $f\circ f=\cos^2$ and $\cos^2|_{[n\frac{\pi}2,(n+1)\frac{\pi}2]}$ is injective, this yields:

Proposition 1 $f|_{[n\frac{\pi}2,(n+1)\frac{\pi}2]}$ is injective.


Proposition 2 $f|_{[n\frac{\pi}2,(n+1)\frac{\pi}2]}$ is strictly monotone.

Proof Suppose $f(n\frac{\pi}2)<f((n+1)\frac{\pi}2)$. We'll show that $f|_{[n\frac{\pi}2,(n+1)\frac{\pi}2]}$ is strictly monotonically increasing.

Take $x,y\in(n\frac{\pi}2,(n+1)\frac{\pi}2)$ such that $x<y$. Suppose $f(x)>f(y)$. Note that $$ f(x)>\max\{f(n\frac{\pi}2),f(y)\}\quad\text{or}\quad f(y)<\min\{f(x),f((n+1)\frac{\pi}2)\} $$ where the left equality holds whenever $f(n\frac{\pi}2)<f(x)$, and the right one whenever $f(y)<f((n+1)\frac{\pi}2)$ (there's some overlap, but this has no influence on the proof). In the 'left' case, by the IVP, $f$ must touch $\max\{f(n\frac{\pi}2),f(y)\}$ at least twice on $[n\frac{\pi}2,y]$, in contradiction to injectivity. In the 'right' case, the same applies to $\min\{f(x),f((n+1)\frac{\pi}2)\}$. Hence necessarily $f(x)<f(y)$.

Analogously, if $f(n\frac{\pi}2)>f((n+1)\frac{\pi}2)$ then $f|_{[n\frac{\pi}2,(n+1)\frac{\pi}2]}$ is strictly monotonically decreasing.


Lemma 2 Let $a,b\in\mathbb{R}$, and $f:[a,b]\rightarrow\mathbb{R}$ be a strictly monotone function with the IVP. Then $f$ is continuous.

Proof Without loss of generality, assume $f$ is increasing; otherwise take $-f$, since 'taking the negative' doesn't influence continuity. Choose $x\in[a,b]$ and $\epsilon>0$. Note that, by the IVP, there exist $x_-,x_+\in[a,b]$ such that $$ f(x_-)=\min\{f(x)-\epsilon, f(a)\} \quad \text{and} \quad f(x_+)=\min\{f(x)+\epsilon, f(b)\}$$ Set $\delta=\min\{|x-x_-|,|x-x_+|\}\backslash\{0\}$ (i.e. we ignore the $x_-$ or $x_+$ term if $x_-=f(a)$ or $x_+=f(b)$, respectively). Then, for any $y\in(x-\delta,x+\delta)\cap[a,b]$, by monotonicity, $$ f(x_-)\le f(y)\le f(x_+) $$ hence $$ f(x)-\epsilon<f(y)<f(x)+\epsilon\text. $$

Take $a=n\frac{\pi}2$, $b=(n+1)\frac{\pi}2$. Then $f|_{[n\frac{\pi}2,(n+1)\frac{\pi}2]}$ is continuous. In particular, $f(x)$ is both left continuous and right continuous at $x=n\frac{\pi}2$, for any $n\in\mathbb{Z}$, and we get:

Proposition 3 $f$ is continuous.

Note that, for any $x\in\mathbb{R}$, $$ f(\cos^2(x)) = \cos^2(f(x)) \in [0,1] $$ Thus we can define the (continuous) function $\tilde f:\mathbb{R}\rightarrow[0,\pi]$ by $$ \tilde f(x)=\arccos\sqrt{f(\cos^2(x))} $$ It is easy to see that $g$ is also a solution: $$ \tilde f(\tilde f(x)) = \arccos\sqrt{f(\cos^2(\arccos\sqrt{f(\cos^2(x))}))} \\ = \arccos\sqrt{f(f(\cos^2(x)))} \\ = \arccos\sqrt{\cos^2(\cos^2(x))} \\ = \cos^2(x) $$ Without loss of generality, assume henceforth that $f=\tilde f$. This gives:

Proposition 4

  • $f$ is nonnegative
  • $f$ is even
  • $f$ is periodic with period $\pi$.

Note that $f(f(\frac{\pi}2))=0$, so $0\in\operatorname{im}f$ ; by periodicity and monotonicity, $f$ attains the value $0$ at one endpoint of each interval $[n\frac{\pi}2,(n+1)\frac{\pi}2]$. Focusing on the interval $[0,\frac{\pi}2]$, we have two cases:

  1. $f(0)=0$, or
  2. $f(\frac{\pi}2)=0$

If (1.), then $$ 0 = f(0) = f(f(0)) = \cos^2(0) = 1 \text; $$ if (2.), then $$ 0 < f(0) = f(f(\frac{\pi}2)) = \cos^2(\frac{\pi}2) = 0 \text; $$ both contradictions. We conclude that no such $f$ exists.

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i completed my answer, feel free to comment on it –  suissidle Apr 6 '13 at 12:45
An injective monotone function is clearly strictly monotone. So in propositon 2 it may be simpler to prove $f$ is just monotone. Also lemma 2 can be simplifies using the fact that a monotone function has countable number of (jump) discontinuities. –  user59671 Apr 9 '13 at 10:06
in the proof of prop 2 i'm just assuming injectivity throughout (my usage of $<$ instead of $\le$ etc); i know it's worded in a convoluted way but that's because it summarizes the cases to be checked; i don't see how dropping the 'strictness' would make it simpler. as to lemma 2, i don't see why it matters how many discontinuities there are, although it's important to see that all discontinuities are jumps; in the proof i implicitly showed the existence of left and right limits. –  suissidle Apr 9 '13 at 10:32
btw i only realized the utility of $\tilde f$ later on, and maybe all the workup to show continuity of $f$ isn't necessary (just show that $\tilde f$ has the IVP, or that the IVP is conserved by composing with continuous and injective functions) although it was perversely interesting to find out all the properties $f$ (which doesn't exist anyway) has to satisfy –  suissidle Apr 9 '13 at 10:33

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