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I came across this graphic method for checking divisibility by $7$.

$\hskip1.5in$ enter image description here

Write down a number $n$. Start at the small white node at the bottom of the graph. For each digit $d$ in $n$, follow $d$ black arrows in a succession, and as you move from one digit to the next, follow $1$ white arrow.

For example, if $n = 325$, follow $3$ black arrows, then $1$ white arrow, then $2$ black arrows, then $1$ white arrow, and finally $5$ black arrows.

If you end up back at the white node, $n$ is divisible by $7$.

This method indeed works, I was wondering why this work and how could we generate such graph for an arbitrary integer $k$?

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4 Answers 4

up vote 14 down vote accepted

Suppose you want to make a divisibility graph for $n$. Make $n$ dots and label them $0, 1, \ldots n-1$.

You will have a "current number" which will change as you walk around the graph. You will be at dot $i$ if the remainder you get from dividing the "current number" by $n$ is $i$. As you walk around the graph, the current number will change, and the dot will change to track the remainder from dividing the new current number. Initially, the "current number" is $0$, so you start at dot $0$.

Black arrows represent the operation of adding 1 to the current number. Make a black arrow from each dot $i$ to $i+1$, and also from dot $n-1$ to dot $0$.

White arrows represent the operation of multiplying the current number by 10. Make a white arrow from each dot $i$ to $10\cdot i\bmod n$. That is, from $i$ to whatever is the remainder is when you divide $10\cdot i$ by $n$. For example, if $i$ is 2 and $n$ is 7, you make a white arrow from dot 2 to dot 6 because the remainder after dividing $2\cdot10$ by $7$ is 6.

Any current number can be generated starting from 0 by adding 1 and multiplying by 10 in the right order. For example, to get to 213, you start at 0 and:

$$ 0\stackrel{+1}\to1\stackrel{+1}\to2 \stackrel{\times10}\to20\stackrel{+1}\to21 \stackrel{\times10}\to210\stackrel{+1}\to211\stackrel{+1}\to212\stackrel{+1}\to213$$

Starting at dot 0 and following the arrows gives you, at each stage, the result of dividing the current number by $n$. The black arrows track the remainder if you add 1 to the current number, and the white arrows track what happens to the remainder if you multiply the current number by 10.

As you traverse the graph, you are keeping track of what the remainder would be at each stage if you divided the current number by $n$. After you've finished building up the current number into the number you want to check, the result is divisible by $n$ only if the remainder would be 0—that is, only if you're back at dot $0$ where you started.

Here's an example, the corresponding graph for $n=8$. I colored the ×10 arrows blue instead of white, because blue is pretty:

divisibility graph for n=8

I made two sets of these, for $2\le n\le 20$, which are viewable online; please enjoy them and do what you like with them.

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I thought I would add a note to some of the answers above.

I have redrawn it with a different layout. It's nice because it shows the symmetry.

The black arrows, which correspond to $+1$ move round the edge. The purple arrows correspond to multiplying by $10$. The nodes are labeled with the remainder after division by 7.

enter image description here

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Hint: The black steps add $1\pmod 7$ and the white steps multiply by $\,10\equiv 3\pmod 7.\,$ Following the path evaluates the number in Horner form $\, ((\cdots d_n) 10 + \cdots + d_1) 10 + d_0.$ If you trace the black arrows starting from the white node $\equiv 0\,$ you'll successively find the nodes for $\,0,1,\ldots 6.\,$ Then you can verify that the white arrows indeed multiply by $\,3\equiv 10\pmod 7.\,$ For example, the node $\equiv 1\,$ is one black step from the start node, and we see that taking one white step from $\,1\,$ leads to the node that is two black steps from $\,1,\,$ i.e the node $\equiv 3,\,$ which means that $\,10\cdot 1\equiv 3\pmod 7.\,$ The graph is that of a finite state machine that evaluates the integer mod $7,\,$ by evaluating the radix polynomial in nested Horner form.

Let's work out your example, $\,325 = ((\color{#c00}3)10+\color{#0a0}2)10 + \color{blue}5,\ $ so we follow $\,\color{#c00}3\,$ black arrows to get $\,0\!+\! 1\! +\! 1\! +\! 1\equiv 3.\,$ Then we follow a white arrow to get $\, (3)10 = 30.\,$ Then $\,\color{#0a0}2\,$ black arrows to get $\,30+2 = 32\,$. Then a white arrow to get $\,(32)10 = 320.\,$ Then, finally, $\,\color{blue}5\,$ black arrows to get $\,320+5 = 325,\,$ where all calculations are done mod $\,7\,$ by the machine.

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Yes, you can find such graphs for divisibility by any number in any base.

This is a finite-state machine representation of the number, similar to what you would get if you performed long division but ignored the quotient so far. (This particular one may have simplifications that obscure that connection.)

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