Calkin-Wilf Tree

I gave this talk at CMI STEMS final camp 2024.

Definitions

Before proceeding, we must be clear about what our title means.

What do you mean by Counting?

What do we mean by the term counting? We are going to prove that Rational numbers are countable. That is, there is a bijection between natural numbers and rational numbers.

A bijective function $f:X\rightarrow Y$ is a one-to-one (injective) and onto (surjective) mapping of a set $X$ to a set $Y$.

Note that every bijection from set $X$ to a set $Y$ also has an inverse function from set $Y$ to set $X$. But how are we going to create the bijection? We will first create a bijection between the Natural numbers and Positive rationals. Let $f(1),f(2),\dots $ be the mapping from natural numbers from $\Bbb{N}\rightarrow +\Bbb{Q}$. Then, note that there is also a bijection from $\Bbb{N}\rightarrow -\Bbb{Q}$ by simply mapping $i\in \Bbb{N}\rightarrow -f(i)$. And to create the bijection from $g:\Bbb{N} \rightarrow \Bbb{Q}$, consider \begin{equation} g(n)= \begin{cases} f(k) & \text{if } n=2k, k\in \Bbb{N}\\ -f(k) & \text{if } n=2k+1, k\in \Bbb{N}, k>1\\ 0 & \text{if } n=1 \end{cases} \end{equation} Great! So we know how to create a bijection between $\Bbb{N}\rightarrow\Bbb{Q}$ if we are given the bijection between $\Bbb{N}\rightarrow+\Bbb{Q}$. But how can we create the bijection between $\Bbb{N}\rightarrow+\Bbb{Q}$? We use the Cute tree!

Cute tree

Every node of this binary tree is assigned a positive fraction.And the starting node is $\frac{1}{1}$. If the positive fraction is $\frac{p}{q}$, then we define it's left child as $\frac{p}{p+q}$ and the right child as $\frac{p+q}{q}$. We call the fraction $\frac{p}{q}$ as the parent of $\frac{p}{p+q}$ and $\frac{p+q}{q}$.

Introduction

In general, if we have the fraction as $x$, then note that it's left child is $\frac{x}{x+1}$ and the right child is $x+1$. However, before proceeding, we shall prove this.

If the parent is $x$, then it's left child is $\frac{x}{x+1}$ and the right child is $x+1$.

Let $x=\frac{p}{q}$. Then by definition, $$\text{ the right child is }\frac{p+q}{q}=\frac{p}{q}+1=x+1. $$ And by definition, $$\text{ the left child is }\frac{p}{p+q}=\frac{1}{\left(\frac{p+q}{q}\right)}\cdot \frac{p}{q}=\frac{x}{x+1}.$$

Infinite cute tree

We start with $\frac{1}{1}$ and proceed to make the left child and right child. Note that this process is infinite and hence the name.

$$\vdots$$

Define the $1$st row has $1/1$. The second row has childs of $1/1$. The $n+1$st row is the row which has childs of $n$th row fractions.

Some properties

Here are a few properties of the infinite cute tree which we will proceed to prove in the talk.

Every positive rational number appears in the tree and appears uniquely on the tree.

Assuming this proposition is true, we can simply number every node in every row systematically. But how are we numbering? Start numbering from the first row and then number starting from the right side of the next row and continue. We get $$f(1)\rightarrow \frac{1}{1}$$ $$f(2)\rightarrow \frac{1}{2}$$ $$f(3)\rightarrow \frac{2}{1}$$ $$f(4)\rightarrow \frac{1}{3}$$ $$f(5)\rightarrow \frac{3}{2}$$ $$\vdots$$

All the fractions in the infite cute tree are in reduced form.

We can prove this using induction.

If $\frac{p}{q}$ is reduced then so is $\frac{p}{p+q},\frac{p+q}{q}$.

Note that $$(p,q)=1\implies (p+q,p)=1,(q,p+q)=1\implies \frac{p}{p+q},\frac{p+q}{q}\text{ are in reduced form if }$$ $\frac{p}{q}\text{ is in reduced form }. $ Clearly elements of row $1$ are reduced. Say elements are reduced for $k$th row. Then by our above claim, we get that $k+1$th row is reduced, and by induction, we are done.

The left child of any vertex is always strictly less than $1$.

Let $\frac{p}{q}$ is the parent, then the left child is $\frac{p}{p+q}$ which is less than $\frac{p}{p}=1$.

For example, the left child of $\frac{20}{23}$ is $\frac{20}{43}$ which is less than $1$.

The right child is always strictly greater than $1$.

Let $\frac{p}{q}$ is the parent, then the right child is $\frac{p+q}{q}$ which is greater than $\frac{q}{q}=1$.

For example, the right child of $\frac{20}{23}$ is $\frac{43}{23}$ which is greater than $1$.

Every vertex is the product of its childs.

For any node, the childs are $\frac{p}{q}$ are $\frac{p}{p+q}$ and $\frac{p+q}{q}$. And the clearly, the product of the children are $\frac{p}{p+q}\times \frac{p+q}{q}= \frac{p}{q}$.

Cool sequence Algorithm

This algorithm can be used to find the unique path from any given reduced fraction to $1/1$. Moreover, this algorithm also proves proposition. \begin{equation} \text{For any fraction } a/b, \text{ the parent is} \begin{cases} a/b-a & \text{if } b>a\\ a-b/b & \text{if } a>b \end{cases} \end{equation} For example, take any fraction, $\frac{13}{47}$ then the parent of $\frac{13}{47}$ is $\frac{13}{34}$. We will now trace it back to $1/1$ as all fractions originate from $1/1$. So we have $$13/47\rightarrow 13/34\rightarrow 13/21\rightarrow13/8 \rightarrow5/8\rightarrow5/3\rightarrow 2/3\rightarrow2/1\rightarrow1/1. $$

Let Cool sequence path denote the path which take any fraction $p/q$ to $1/1$.

Note that for any fraction which appears in the tree will have a unique parent and hence a unique cool sequence path. Hence, any fraction appearing in the cute tree appears only once. Now, we claim that any reduced fraction appears on this tree. However, note that our algorithm works for any reduced fraction.

More Properties

Number of elements in $n$th row is $2^{n-1}$.

We can prove this by induction. The base case is true. Suppose it's true for $k$th row, then by definition of $k+1$th row, the $k+1$th row will contain childs of $k$th row. Note that these childs will be unique and distinct by the first proposition, we get that the number of elements in the $k+1$th row is $2$ times the number of elements in the $k$th row, which by induction is $2^{k-1}$. And we are done.

The $i$th node from the left in any given row is the reciprocal of $i$th vertex from the right of the tree of that row.

We proceed with induction. True for the first row. Say it is true for the $k$th row. Then say the $i$th node from the left is $p/q$ and the $i$th node from the left is $q/p$. Then we consider the children. The right child of $p/q$ is the $2i-1$th from left in row $k+1$ and the left child of $q/p$ is the $2i-1$th from right in row $k+1$. However, The right child of $p/q$ is $p/p+q$ and the left child of $q/p$ is $q+p/p$, and both are reciprocal of each other. Similarly, the left child of $p/q$ and the right child of $q/p$ are the $2i$th element from left and right, respectively, in row $k+1$. And the left child of $p/q$ is $p/p+q$ and the right child of $q/p$ is $p+q/p$, which are clearly reciprocal of each other. And our hypothesis is true for $k+1$th row. So, we are done by induction!

The product of all the elements in a given row is $1$.

True for the first row. For any $k>1$th row, we get that there are even number of elements. Using the above proposition, we get that every $i$th element from the left can be paired with the $i$th element from the right. Note that both these elements are reciprocal of each other, and hence the product of them is $1$. And hence the product is $1$.

The $1$st node from the left in $n$th row is $1/n$ and the $1st$ node from right in $n$th row is $n/1$.

True for the first row. Say it is true for the $k$th row. Then the leftest node must be $1/k$ and rightest node is $k/1$. Note that the leftest node in the $k+1$th row is the left child of the leftest node in $k$th row. Hence it must be $1/(k+1)$. Similarly, we get that the rightest node in the $k+1$th row is the right child of the rightest node in $k$th row. Hence it must be $(k+1)/1$.

Sum of all elements in $n$th row is $3\cdot 2^{n-2}-\frac{1}{2}$.

We proceed with induction. It is true for $n=1$ row. Say it's true for $k$th row. Now consider the $k+1$ row. For any fraction, we know it's reciprocal is there. So for $a/b$ in the $k$th row, we know that $b/a$ is there too. We consider the children which are in the $k+1$th row and the sum. So we get $$a/(a+b)+(a+b)/b+b/(a+b)+(a+b)/a= 3+a/b+b/a.$$ Hence, we get the sum of elements in $k+1$th row is $$3\times 2^{k-2}+3\cdot 2^{k-2}-\frac{1}{2}=3\times 2^{k-1}-\frac{1}{2}.$$ And we are done by induction.

Binary preimage

Using our cool sequence algorithm, we showed that our function $f$ covers all positive rational numbers uniquely. Now, we will show that given any fraction, how we can find its preimage (which is obviously unique). Remember our Cool sequence algorithm? We will do the same thing, except now we will also care about the left and right child. For example, we know that $13/47$ is leftchild of $13/34$ and so on. So we get $$\frac{13}{47}\overset{\text{L}}{\rightarrow} \frac{13}{34}\overset{\text{L}}{\rightarrow} \frac{13}{21}\overset{\text{L}}{\rightarrow}\frac{13}{8} \overset{\text{R}}{\rightarrow}\frac{5}{8}\overset{\text{L}}{\rightarrow}\frac{5}{3}\overset{\text{R}}{\rightarrow}\frac{2}{3}\overset{\text{L}}{\rightarrow}\frac{2}{1}\overset{\text{R}}{\rightarrow}\frac{1}{1}. $$ We define a new function, $g(n)$ which is basically the same as $f(n)$ but we define $g(n)$ as the following.

Note that $$f(n)=g(2^{\log_2(n)}+2^{\log_2(n)+1}-1-n).$$ Note that if the number $g(n)$ is the parent, then its right child is $g(2n)$ and the left child is $g(2n+1)$. Now consider Now, consider the binary representation of $n,2n$ and $2n+1$. If the $n=(a_1a_2\dots a_k)_2$ then note that $2n=(a_1\dots a_k0)_2$ and $2n+1=(a_1\dots a_k1)_2$. So we add $1$ at the end of the binary reprsentation whenever we go to the left child and $0$ if right. Hence, for the above example, we start with $1/1$. Since $2/1$ is right child, we add $1$ to binary expression. Hence $(11)_2$. And we proceed like this. We get that $$13/47=g((110101000)_2)=g(424)=f(512+256-1-424)=f(343).$$ Note that, using our above results, we get that there is a bijection between naturals and positive rationals. And hence by our discussion in \textbf{section 1.2}, we get that there is a bijection between naturals and rational numbers. Hence, rationals are countable.

Connection with Euclidean algorithm

We consider $13/47$ as the example. Recall, we had $$\frac{13}{47}\overset{\text{L}}{\rightarrow} \frac{13}{34}\overset{\text{L}}{\rightarrow} \frac{13}{21}\overset{\text{L}}{\rightarrow}\frac{13}{8} \overset{\text{R}}{\rightarrow}\frac{5}{8}\overset{\text{L}}{\rightarrow}\frac{5}{3}\overset{\text{R}}{\rightarrow}\frac{2}{3}\overset{\text{L}}{\rightarrow}\frac{2}{1}\overset{\text{R}}{\rightarrow}\frac{1}{1}. $$ Note that $$47=3\times 13+8\text{ hence three times L}$$ $$13=1\times 8+5\text{ hence one time R}$$ $$8=1\times5+3\text{ hence one time L}$$ $$5=1\times 3+2\text{ hence one time R}$$ $$3=1\times 2+1\text{ hence one time L}$$ $$2=2\times 1+0$$ Note that at every step, our $L,R$ are alternating. This is because if $a=qb+r$ was say $L$, then that implies the fraction was $b/a$ and then after $qL$'s we get to $b/r$, but we have $b>r$ by euclidean algorithm. So we get that $b/r$ must be a right child. Hence they are alternating. Moreover, whenever we get the equation of the form $n=n\times 1+0$ at the end of the euclidean algorithm ( note that we are supposed to get this as the fractions are in reduced form), we write $n-1$ times the symbol. Hence, for our current example, we have $n-1=2-1=1$ times $R$. Since we can connect euclidean algorithm and finite continued fractions, we can also connect the fractions with finite continued fractions.

Address formula

Using the binary preimage, we can now figure out the actuall address for any fraction. For any fraction $a/b$, we figure out the $n$ such that $f(n)=a/b$. The the row $f(n)$ belongs to is ${[\log_2(n)]}+1$ as in the $k$th row, $f(2^{k-1}),\dots, f(2^{k}-1)$ are there. And then the fraction would be $n-2^{[\log_2(n)]}$th element from left.

Algebraic formula

We claim the following.

We have the following recursive formula $$f(n+1)=\dfrac{1}{[f(n)]+1-\{f(n)\}}$$ with $f(1)=1/1=1$.

We firstly simplify the formula.

Note that $$f(n+1)=\dfrac{1}{[f(n)]+1-\{f(n)\}}=\dfrac{1}{2[f(n)]+1-f(n)}.$$

As we have $f(n)=[f(n)]+\{f(n)\}$, we get that $$[f(n)]+1-\{f(n)\}=[f(n)]+1-(f(n)-[f(n)])=2[f(n)]+1-f(n).$$

Note that for any fraction $f(n+1)$ where $f(n+1)$ is the left child, we can say that $f(n+1)=\dfrac{1}{[f(n)]+1-\{f(n)\}}$.

For any fraction $f(n+1)$ where $f(n+1)$ is the left child, we can say that $f(n+1)=\dfrac{1}{2[f(n)]+1-f(n)}$

We get that the left child is $f(n)=\frac{x}{x+1}$ and right child is $f(n+1)=x+1$ for any vertex $x$. We have to show that $$\dfrac{1}{2[f(n)]+1-f(n)}=f(n+1)$$ or show that $$2f(n+1)[f(n)]+f(n+1)-f(n)f(n+1)=1.$$ But note that $f(n)$ is a left child, so we get that $2f(n+1)[f(n)]=0.$ Moreover, we get that $$f(n+1)-f(n)f(n+1)=x+1-x=1.$$ And we are done!

So our theorem holds true whenever $f(n)$ is a left child. Now, we see the case when $f(n)$ is the right child. Since $f(n)$ is the right child, we have $f(n+1)$ is the left child of some other parent. Since all fractions are getting generated by $1/1$, the fraction $f(n),f(n+1)$ must have same parent some rows ago. Let $a/b$ be that common parent fraction $k+1$ rows. Then note that $f(n)$ is generated by taking $k$ consecutive right childs after one left child from $a/b$ and $f(n+1)$ is generated by taking $k$ consecutive left child after one right child from $a/b$. Now, before proceeding further, we present a lemma.

For any number $x = p/q$, the rightmost child after $n$ rows is $(p + nq)/q = x + n$, and the leftmost child is $p/(np + q) = x/(nx + 1)$.

The proof is just induction. It is true for $n=1$ case. Say it is true for $n=k$. Then note that the right child of rightmost child in $k$th row is rightmost child in $k+1$th row. And right child is $(p+(k+1)q)/q=x+k+1$. And similarly for left child. And we are done by induction.

The left child of $a/b$ is $a/(a+b)$ and right child is $(a+b)/b$. As $f(n)$ is is generated by taking $k$ consecutive right childs after one left child from $a/b$, by above lemma, we have $$f(n)=a/(a+b)+k$$ and we have $$f(n+1)=[(a+b)/b]/[k(a+b)/b+1]$$ $$=(a + b)/(k(a + b) + b)=1/(k + b/(a+b)).$$ Now, note that $[f(n)]=k$. And hence we have $$f(n)-k=a/(a+b)\implies f(n)-[f(n)]=a/(a+b).$$ So note that $$f(n+1)=\frac{1}{1-a/(a+b)+k}=\frac{1}{1-f(n)+2k}=\frac{1}{1-f(n)+2[f(n)]}.$$ Hence, we prove for the right child too. I hope you liked reading it!