# 2020 CIME II Problems/Problem 9

## Problem 9

Let $f(x)=x^2-2$. There are $N$ real numbers $x$ such that $$\underbrace{f(f(\ldots f}_{2019\text{ times}}(x)\ldots))=\underbrace{f(f(\ldots f}_{2020\text{ times}}(x)\ldots)).$$Find the remainder when $N$ is divided by $1000$.

## Solution 1

Let $f^n(x)$ denote the composition of $f$ with itself $n$ times. We require that $f^{2019}(x)=f^{2020}(x)$. Since

$f^{2020}(x)=f(f^{2019}(x))=(f^{2019}(x))^2-2$, we have $f^{2019}(x)=(f^{2019}(x))^2-2$. Hence

$(f^{2019}(x)+1)(f^{2019}(x)-2)=0$ so that $f^{2019}(x)\in\{-1,2\}$.

Now if $f^{n+1}(x)=k$ for some $k$, then $k=f^{n+1}(x)=f(f^{n}(x))=(f^{n}(x))^2-2$ so that

$f^{n}(x)=\pm\sqrt{k+2}$. Hence we find $f^{2018}(x)\in\{\pm1,\pm2\}$, and similarly

$f^{2017}(x)\in\{\pm\sqrt{3},\pm1,\pm2,0\}$. Note that at each step since our values for $k$ are $\geq -2$ we have

$\sqrt{k+2}$ is real; and $\pm\sqrt{k+2}$ are distinct, and also $\neq k$, except when $k=-2$. Note that every $x$ that satisfies $f^{n+1}(x)=k$ also satisfies $f^{n}(x)=\pm\sqrt{k+2}$. Since $-2$ is a value of $k$ starting with $f^{2018}$, we have for $n\leq 2018$, $f^{n}(x)$ must equal any one of $3\cdot 2^{2018-n}+1$ many values. It follows that there are $3\cdot 2^{2018}+1$ many real solutions.

By Eulers Theorem $2^{\phi(125)}=2^{100}\equiv1\mod 125$ so $2^{2000}=(2^{100})^{20}\equiv 1^{20}=1\mod 125$ and hence $2^{2003}\equiv 8\mod 1000$. Since $2^{15}=2^{10}\cdot 2^5\equiv 24\cdot 32=768\mod 1000$ we have $2^{2018}\equiv 8\cdot 768\equiv 144\mod 1000$. Hence $3\cdot 2^{2018}+1\equiv 3\cdot 144+1=433\mod 1000$. Thus the answer is 433 as desired.

## Solution 2

We can start by finding the number of solution for smaller repeptitions of $f$. Notice that we can solve $f(f(x))=f(x)$ by applying the functional inverse $f^{-1}$ to both sides as you would to solve any equation: $f^{-1}(f(f(x)))=f^{-1}(f(x))\Longrightarrow f(x)=|x|$ (We put the absolute value bars because we know that taking the inverse of $f$ of both sides involves taking the square root of both sides, and $\sqrt{t^2}=|t|$). From here, it is easy to see that this equation has $4$ solutions at $\pm1$ and $\pm2$. We can also try for $f(f(f(x)))=f(f(x))$ (we will solve more methodically here): $$((x^2-2)^2-2)^2-2=(x^2-2)^2-2$$ $$(x^2-2)^2-2)^2 = (x^2-2)^2$$ $$(x^2-2)^2-2=|x^2-2|$$ $$(x^2-2)^2-2=\pm(x^2-2)$$ $$(x^2-2)^2=x^2 \textup{ OR } (x^2-2)^2=-x^2+4$$The first equation yeilds $4$ results, and the second equation yields $2$ results for a total of $6$ results. It appears that $\underbrace{f(f\cdots f}_{n\textup{ times}}(x))=\underbrace{f(f\cdots f}_{n-1\textup{ times}}(x))$ bas $2n$ real solutions, giving a total of $4040$ apparent solutions for the original equation. This makes logical sense considering that $f$ is an even polynomial with 2 roots. For a more formal proof, we consider $F_n(x)=\underbrace{f(f\cdots f}_{n\textup{ times}}(x))$. We are asked to find the number of solutions of the equation in the form $F_n(x)=F_{n-1}(x)$. Following from how we solved the first simple case, $f^{-1}(F_n(x))=f^{-1}(F_{n-1}(x)) = F_{n-1}(x)=|F_{n-2}(x)|$. Note that the absolute value branches off in rwo directions: $F_{n-1}(x)=\pm F_{n-2}(x)$. This would give a total of $2\cdot2n=4n$ real and complex solutions (we multiply by 2 because $f$ is a quadratic, which has 2 total roots). The complex roots come from the negative branches, so there are $2\cdot n=2n$ complex solutions. Therefore, there are a total of $2n$ real roots, which again gives $4040$ roots for the original question. The question asks for $\boxed{040}\equiv4040(\mod1000)$~bhargavakanakapura

## Solution 3

Given the function $f(x)=x^2-2$, we need to find the number of real numbers $x$ that satisfy: $$f^{2019}(x)=f^{2020}(x)$$

Here, $f^n(x)$ denotes the function $f$ iterated $n$ times. To proceed, we note that: $$f^{2020}(x)=f\left(f^{2019}(x)\right)$$

Thus, the equation becomes: $$f^{2019}(x)=f\left(f^{2019}(x)\right)$$

Let $y=f^{2019}(x)$. Then we need: $$y=f(y)$$

Given $f(y)=y^2-2$, the equation $y=y^2-2$ can be rewritten as: $$y^2-y-2=0$$

Solving this quadratic equation: $$y^2-y-2=(y-2)(y+1)=0$$

Thus, $y$ can be either 2 or -1 . Hence, we must solve for $x$ such that: 1. $f^{2019}(x)=2$ 2. $f^{2019}(x)=-1$

First, consider $f^{2019}(x)=2$. We will determine the structure of the iterates of $f$ to see how many $x$ lead to 2 . Next, consider $f^{2019}(x)=-1$ and analyze similarly.

We start by analyzing the dynamics of $f(x)$. The fixed points of $f(x)$ are the solutions to $x=f(x)$ : $$x=x^2-2 \Rightarrow x^2-x-2=0$$

The solutions are $x=2$ and $x=-1$.

Next, we analyze the behavior around these fixed points: - For $y=2, f(2)=2^2-2=2$. So 2 is a fixed point of $f$. - For $y=-1, f(-1)=(-1)^2-2=-1$. So -1 is also a fixed point of $f$.

Now we determine the number of preimages for these points under $f$. Given $y=2$ : $$f(x)=2 \quad \Rightarrow \quad x^2-2=2 \quad \Rightarrow \quad x^2=4 \quad \Rightarrow \quad x= \pm 2$$

Given $y=-1:$ $$f(x)=-1 \quad \Rightarrow \quad x^2-2=-1 \quad \Rightarrow \quad x^2=1 \quad \Rightarrow \quad x= \pm 1$$

At each iteration, each of $\pm 2$ and $\pm 1$ can have 2 preimages under the iteration. This means the total number of solutions doubles with each iteration. - For $f^{2019}(x)=2$, starting with 2, it could be reached through $f^{2018}(x)= \pm 2$ with doubling at each iteration. - For $f^{2019}(x)=-1$, starting with -1 , it could be reached through $f^{2018}(x)= \pm 1$ with doubling at each iteration.

Each iteration brings doubling of the number of preimages, leading to: $$2^{2019} \text { for each target value }(2 \text { and }-1)$$

Summing these: $$N=2 \times 2^{2019}=2^{2020}$$

Finally, we find the remainder when $2^{2020}$ is divided by 1000 . Using Euler's theorem for finding $2^{2020} \bmod 1000$ : $$\phi(1000)=400$$

Thus, $$2^{400} \equiv 1 \quad \bmod 1000$$

Since $2020=5 \times 400+20$ $$2^{2020}=\left(2^{400}\right)^5 \cdot 2^{20} \equiv 1^5 \cdot 2^{20} \equiv 2^{20} \quad \bmod 1000$$

Computing $2^{20} \bmod 1000$ : $$2^{10}=1024 \Rightarrow 1024 \quad \bmod 1000=24 \\ 2^{20}= \left(2^{10}\right)^2=24^2=576$$

Thus, the remainder when $N$ is divided by 1000 is: $$\boxed{\text{576}}$$ By AnYu