By Hans-Peter Stricker


2019-01-11 16:31:31 8 Comments

Draw the numbers $1,2,\dots,N$ on a circle and draw a line from $n$ to $m>n$ when $n$ divides $m$:

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For larger $N$ some kind of stable structure emerges

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which remains perfectly in place for ever larger $N$, even though the points on the circle get ever closer, i.e. are moving.

enter image description here enter image description here

This really astonishes me, I wouldn't have guessed. Can someone explain?


In its full beauty the case $N=1000$ (cheating a bit by adding also lines from $m$ to $n$ when $(m-N)\%N$ divides $(n-N)\%N$ thus symmetrizing the picture):

![enter image description here


Note that a similar phenomenon – stable asymptotic patterns, esp. cardioids, nephroids, and so on – can be observed in modular multiplication graphs $M:N$ with a line drawn from $n$ to $m$ if $M\cdot n \equiv m \pmod{N}$.

For the graphs $M:N$, $N > M$ for small $M$

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But not for larger $M$

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For $M:(3M -1)$

![![enter image description here

It would be interesting to understand how these two phenomena relate.


Note that one can create arbitrary large division graphs with circle and compass alone, without even explicitly checking if a number $n$ divides another number $m$:

  1. Create a regular $2^n$-gon.

  2. Mark an initial corner $C_1$.

  3. For each corner $C_k$ do the following:

    1. Set the radius $r$ of the compass to $|C_1C_k|$.

    2. Draw a circle around $C_{k_0} = C_k$ with radius $r$.

    3. On the circle do lie two other corners, pick the next one in counter-clockwise direction, $C_{k_1}$.

    4. If $C_1$ does not lie between $C_{k_0}$ and $C_{k_1}$ (in counter-clockwise direction) or equals $C_{k_1}$:

    5. Draw a line from $C_k$ to $C_{k_1}$.

    6. Let $C_{k_0} = C_{k_1}$ and proceed with 5.

    7. Else: Stop.


There are three equivalent ways to create the division graph for $N$ edge by edge:

  1. For each $n = 1,2,...,N$: For each $m\leq N$ draw an edge between $n$ and $m$ when $n$ divides $m$.

  2. For each $n = 1,2,...,N$: For each $k = 1,2,...,N$ draw an edge between $n$ and $m = k\cdot n$ when $m \leq N$.

  3. For each $k = 1,2,...,N$: For each $n = 1,2,...,N$ draw an edge between $n$ and $m = k\cdot n$ when $m \leq N$.

2 comments

@Hans-Peter Stricker 2019-01-14 09:21:10

Putting the pieces together one may explain the pattern like this:

  • The division graph for $N$ can be seen as the sum of the multiplication graphs $G_N^k$, $k=2,3,..,N$ with an edge from $n$ to $m$ when $k\cdot n = m$. When $n > N/k$ there's no line emanating from $n$. (This relates to step 7 in the geometric construction above.)

  • The multiplication-modulo-$N$ graphs $H_{N}^k$ have a weaker condition: there's an edge from $n$ to $m$ when $k\cdot n \equiv m \pmod{N}$.

  • So the division graph for $N$ is a proper subgraph of the sum of multiplication graphs $H_{N+1}^k$.

  • The multiplication graphs $H_{N}^k$ exhibit characteristic $k-1$-lobed patterns:

enter image description hereenter image description hereenter image description here

  • These patterns are truncated in the graphs $G_{N}^k$ exactly at $N/k$.

  • Overlaying the truncated patterns gives the pattern in question.

@Hans-Peter Stricker 2019-01-12 08:41:24

To add some visual sugar to Alex R's comment (thanks for it):

enter image description here enter image description here enter image description here enter image description here

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