## Radix Trickery

*Harley Wiltzer, December 2022*

This post demonstrates some neat tricks where representing numbers via different radixes can lead to nice simplifications. An example of such a trick is the infamous Radix Sort algorithm, which seemingly sorts numbers in linear time (fine print needed). Here, we will see some other (simpler) radix magic.

To begin, let's recall what we mean by "radix". Usually, when we write numbers, we interpret them as follows:

\begin{equation*} \begin{aligned} 74207281 = 7(10^7) + 4(10^6) + 2(10^5) + 0(10^4) + 7(10^3) + 2(10^2) + 8(10^1) + 1(10^0) \end{aligned} \end{equation*}
The number "10" here is the *radix*, and is fairly arbitrary. In
principle, we can alternatively represent numbers via similar
decompositions replacing 10 with any integer greater than 1. In doing
so, the alphabet of "digits" has to be changed: for radix \(n\), rather
than the digits \(\{0,\dots, 9\}\), we use "\(n\)-gits"
\(\{0,\dots,n-1\}\). When \(n > 10\), this means we need to invent new
symbols. For example, the 12-gits may be written as
\(\{0,1,2,\dots,9,\texttt{A},\texttt{B}\}\) where
\(\texttt{A},\texttt{B}\) represent \(10,11\) respectively. Now, we can
express

where \([x]_{n}\) denotes \(x\) written with respect to radix \(n\).

In the sequel, we will frequently be interested in the sequence of \(n\)-gits that form a radix \(n\) representation of a number. We will express these as follows:

\begin{equation*} \begin{aligned} \quad [x]_n = (x_k) \end{aligned} \end{equation*}which means that the \(n\)-git in the \(n^k\) position of \([x]_n\) is \(x_k\). It is implicit that \(k\) ranges from \(0\) to \(\infty\), where \(x_k = 0\) whenever \(n^k>x\). We will also maintain the assumption that all radixes of interest are small, that is, they themselves are comprised of \(O(1)\) digits.

### Divisibility Tricks

Suppose we want to check if some integer \(x\) is divisible by a natural number \(a\). We'll assume that \(x\) is \(N\) digits long, and \(a\) has \(O(1)\) digits. Naturally, determining if \(a\) divides \(x\) naively takes \(\Theta(N)\) time. Often, we can do better than this. For instance, suppose \(a\in\{1,2,5,10\}\). Then we can actually determine if \(a\) divides \(x\) by checking only the last digit of \(x\) – this requires only \(O(1)\) time. What's special about these numbers? Well, they are precisely the factors of \(10\). So, if instead we used a radix \(b\) with many factors, we should expect to determine divisibility in constant time for a larger set of divisors.

Let \([x]_b = (x_k)\) be given, and suppose \(a\) divides \(b\). Then it only takes \(O(1)\) time to decide if \(a\) divides \([x]_b\). In particular, \(a\) divides \([x]_b\) if and only if \(a\) divides \(x_0\).

Note that \([x]_b = b(x_{k+1}) + x_0 = an + x_0\) for some natural number \(n\), since \(a\) divides \(b\). Then clearly if \(a\) divides \(x_0\), we have \([x]_b = a(n + m)\) for some natural number \(m\), and consequently \(a\) divides \([x]_b\). Conversely, if \(a\) divides \([x]_b\), then \([x]_b = am\) for some natural number \(m\), so

\begin{equation*} \begin{aligned} am &= an + x_0\\ x_0 &= a(m - n) \end{aligned} \end{equation*}and clearly \(a\) divides \(x_0\).

So, given an arbitrarily large number written in radix 12 for instance, we can check if it's divisible by any of \(\{1,2,3,4,6,12\}\) just by checking the last 12-git. We can actually extend this a little bit further.

Let \([x]_b = (x_k)\) be given, and suppose \(a\) divides \(b^n\) for some natural number \(n\geq 1\). Then \(a\) divides \([x]_b\) if and only if \(a\) divides \((x_k)_{k < n}\) (that is, the \(n\) least significant \(b\)-gits of \([x]_b\)).

Similarly to the last proposition, we can write

\begin{equation*} \begin{aligned} \quad[x]_b &= b^n(x_{k+n}) + (x_k)_{k < n} \end{aligned} \end{equation*}The proof follows in the exact same way.

Now, given an arbitrarily large number written in radix 12, we can check if it's divisible by any of \(\{1,2,3,4,6,8,9,12,16,18,24,36,48,72,144\}\) simply by checking the last 2 12-gits. In base 10, this would only work for checking the divisibility of \(\{1,2,4,5,10,25,50,100\}\).

The next trick, in my opinion, is especially cool. Rather than using
only that last few \(n\)-gits to determine divisibility of a number, we will
determine the divisibility by analyzing the *digit sum* of the
number. The digit sum is a quantity that depends on the radix that the
number is represented in. If \([x]_b = (x_k)\), we write the digit sum
of \([x]_b\) as \(\oplus(x_k) = \sum_kx_k\).
We'll begin by
looking at how it manifests itself in decimal, and then generalize.

Let \(x\) be an integer with \(N\) digits and let \(a\in\{3,9\}\). Then \(a\) divides \(x\) if and only if \(a\) divides the digit sum of \(x\).

We will establish this identity by analyzing the difference between a number and its digit sum. Let \(x = (x_k)\). We have

\begin{equation*} \begin{aligned} x - \oplus(x_k) &= \sum_{k=0}^N10^kx_k - \sum_{k=0}^Nx_k\\ &= \sum_{k=0}^Nx^k(10^k - 1)\\ &= \sum_{k=1}^Nx^k(10^k - 1) \end{aligned} \end{equation*}Notice that for any \(k\geq 1\), we have \(10^k -1 = (9)\) (all digits are nines). Clearly, this implies that \(x - \oplus(x_k)\) is a multiple of \(9\), so it is a multiple of \(a\). Therefore, if \(\oplus(x_k)\) is a multiple of \(a\in\{3,9\}\), then \(x\) must also be a multiple of \(a\). Likewise, if \(x\) is a multiple of \(a\in\{3,9\}\), then it must also be the case that \(\oplus(x_k)\) is a multiple of \(a\) – otherwise \(x - \oplus(x_k)\) cannot be a multiple of \(a\).

The significance of this identity is that the digit sum of a number is
*much* smaller than the number itself. In particular, if \(x = (x_k)\) has
\(N\) digits, we have \(\oplus(x_k)\leq 9N\in O(\log x)\). Moreover,
computing the digit sum of a number is much faster than checking for
divisibility naively.

What makes the numbers 3 and 9 special? From the proof, we see that the identity works because these numbers always divide numbers of the form \(999\dots 9\). For an arbitrary radix \(b\), this is equivalent to numbers dividing \(nnn\dots n\), where \(n = b - 1\). This condition would be satisfied for any number \(a\) such that \(b \% a = 1\), where \(\%\) is the remainder operation: \(b\% a\) is the remainder when dividing \(b\) by \(a\).

Let \([x]_b = (x_k)\) be a given integer with \(N\) \(b\)-gits, and let \(a\in\mathbf{N}\) satisfy \(b\%a = 1\). Then \(a\) divides \([x]_b\) if and only if \(a\) divides \(\oplus(x_k)\).

As in the simpler case before, we look at the difference between a number and its digit sum:

\begin{equation*} \begin{aligned} \quad[x]_b - \oplus(x_k) &= \sum_{k=1}^Nx_k(b^k - 1) \end{aligned} \end{equation*}We claim that \(a\) divides \(b^k - 1\) for each \(k\geq 1\). This can be proved simply by induction on \(k\). Our base case corresponds to \(k = 1\). Since \(b\%a = 1\), there exists a natural number \(m\) for which \(b = am + 1\). Thus, \(b - 1 = am\), so clearly \(a\) divides \(b - 1\), which proves the base case. Our induction hypothesis is that \(a\) divides \(b^k - 1\), and it remains to show that \(a\) then divides \(b^{k+1} - 1\). We have

\begin{equation*} \begin{aligned} b^{k+1} - 1 &= b^kb - 1\\ &= (b^k - 1 + 1)b - 1\\ &= (b^k - 1)b + b - 1\\ \end{aligned} \end{equation*}Note that \(a\) divides both \(b^k - 1\) and \(b - 1\) by the induction hypothesis, so it follows that \(a\) divides \(b^{k+1} - 1\), proving the claim.

Then, by the same logic as the decimal case, it follows that \(a\) divides \([x]_b\) if and only if \(a\) divides \(\oplus(x_k)\).

As an example, this tells us that we can reduce the problem of deciding divisibility of a number by 7 to divisibility of the digit sum of the number in its octal (radix 8) representation by 7.

### Geometric Series

The following trick is something I came up with while daydreaming during my undergrad, which I was very proud of. I have since seen another person mention this trick in a Reddit thread, but otherwise I don't believe it is particularly well known. It's also not particularly useful, but it's pretty cool, if I may say so myself.

Recall that a geometric series is a series of the following form

\begin{equation*} \begin{aligned} S_n &= \sum_{k=0}^nb^k \end{aligned} \end{equation*}Usually, we think of \(b\) as a real number. For the purpose of this trick though, it only makes sense to think of \(b\) as a natural number where \(b\geq 2\). The following closed form is well known.

Let \(b\in\mathbf{R}\). Then

\begin{equation*} \begin{aligned} S_n &= \sum_{k=0}^nb^n = \frac{b^{n+1} - 1}{b - 1} \end{aligned} \end{equation*}We will derive an elegant proof of this closed form that applies when \(b\) is a natural number that is at least \(2\). The trick is to express \(S_n\) in radix \(b\). We have

\begin{equation*} \begin{aligned} [S_n]_b &=\sum_{k=0}^n[b^n]_b\\ &= 1 + [10]_b + [100]_b + [1000]_b + \cdots\\ &= (1)_{k\leq n} \end{aligned} \end{equation*}The beauty of this is that \([S_n]_b\) is super easy to mess around with algebraically. Naturally, we have

\begin{equation*} \begin{aligned} (b - 1)[S_n]_b = (b - 1)_{k\leq n} \end{aligned} \end{equation*}We have seen numbers of this form before. In decimal, for example, this is equivalent to numbers of the form \(999\dots 9\). Notably, adding 1 to such a number brings us to \(100\dots 0\), which is \(b^{n+1}\). This means that

\begin{equation*} \begin{aligned} (b - 1)[S_n]_b + 1 &= b^{n+1}\\ S_n = [S_n]_b &= \frac{b^{n+1} - 1}{b - 1} \end{aligned} \end{equation*}While this trick only makes sense when \(b\) can be thought of as a radix, if you ever forget the closed form of the geometric series, you can use this technique to derive this form and prove that it works for any real \(b\) by induction. Personally, I find this radix-based proof much more mechanical than the more common proofs of the closed form of the geometric series.