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In a great video by PBS Infinite Series, the mathematician Kelsey Houston-Edwards argues that bees build their honeycombs into hexagonal shapes because that's the most efficient way of tiling two-dimensional euclidean space that "minimizes the length of lines relative to [the] number of regions".

She goes on in that video to generalize the question and explore possible honeycomb shapes that 4-dimensional bees might produce.

My question is, what is the most efficient shape for tiling non-euclidean 2-dimensional space? Or, in other words, what shape honeycomb would a 3-dimensional bee make in a highly hyperbolic or elliptical space? How much does the curvature of the space effect the tiling? I'd also like to know how this idea generalizes to higher-dimensional curved spaces, if possible.

ZKG
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    I don't know but I know the thought of 4 dimensional killer bees is going to keep me up tonight. – Deepak Aug 04 '17 at 10:24

1 Answers1

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Hyperbolic case

First off, the combinatorics of tilings in hyperbolic geometry will also affect the size of the cells. So I'd guess a hyperbolic bee might be more interested in having approximately bee-sized cells, even if it means using slightly more wax. But that's a question for hyperbolic biology, so back to the geometry.

The way you quoted it, you'd be minimizing line length per number, which has a length unit remaining. Thus the optimum is not invariant under scale. But the video mentions that the cells should have unit area. So if you had a honeycomb with cells of area 9 square length units, you'd scale it down by 3 length units and then start measuring edge lengths. Or in other words, you are minimizing perimeter divided by the square root of the enclosed area. You could as well square that and minimize squared perimeter divided by enclosed area. If you care about absolute numbers, particularly comparing them with the limit process described in the video, you might want to divide the perimeter by two since each wall is shared by two adjacent cells, so the contribution per cell is just half of that. But for the sake of finding the optimum it does not matter.

Let's consider a regular hyperbolic tiling of $n$-gons, $m$ of them meeting at each corner, with $\frac1m+\frac1n<\frac12$. What's its edge length? The hyperbolic law of cosines for curvature $-1$ states

$$\cosh a=\frac{\cos\alpha+\cos\beta\cos\gamma}{\sin\beta\sin\gamma}$$

Now consider a right triangle covering $1/2n$ of your cell. It has $\alpha=\frac\pi n$ at the center, $\beta=\frac\pi m$ at the vertex and $\gamma=\frac\pi 2$ at the center of the edge of the cell. This leads to

$$a=\operatorname{arcosh}\frac{\cos\frac\pi n}{\sin\frac\pi m}$$

The area of that triangle is equal to the angle defect:

$$A=\pi-\frac\pi n-\frac\pi m-\frac\pi 2=\pi\left(\frac12-\frac1n-\frac1m\right)$$

Now the whole cell is composed of $2n$ such triangles, so the total perimeter is $2na$ and the total area is $2nA$. Thus the number to optimize is

$$\frac{(2na)^2}{2nA}=\frac{2n\left(\operatorname{arcosh}\frac{\cos\frac\pi n}{\sin\frac\pi m}\right)^2}{\pi\left(\frac12-\frac1n-\frac1m\right)}$$

Now you can try this out for some values of $m$ and $n$:

$$\begin{array}{c|ccccccc} n\backslash m & 3 & 4 & 5 & 6 & 7 & 8 & 9 \\\hline 3 & & & & & 23.8496 & 26.7747 & 29.5759 \\ 4 & & & 20.0136 & 23.7379 & 27.2316 & 30.5319 & 33.6653 \\ 5 & & 17.9257 & 22.5929 & 26.8885 & 30.8947 & 34.6616 & 38.2245 \\ 6 & & 19.8745 & 25.2172 & 30.1103 & 34.6579 & 38.9226 & 42.9479 \\ 7 & \mathbf{15.0035} & 21.8342 & 27.8626 & 33.3653 & 38.4677 & 43.2445 & 47.7470 \\ 8 & 16.1524 & 23.7997 & 30.5195 & 36.6382 & 42.3027 & 47.5994 & 52.5871 \\ 9 & 17.3024 & 25.7687 & 33.1832 & 39.9220 & 46.1528 & 51.9740 & 57.4518 \end{array}$$

It turns out the optimum is the 7,3 tiling: regular heptagons, three of them meeting at each vertex. This is also the tiling with the smallest cells, so if the cells aren't too small for our hyperbolic bees, they should pick this one. For comparison, the Euclidean hexagon is at $8\sqrt3\approx13.8564$, the Euclidean square at $16$. So that hyperbolic heptagonal tiling is still better than the Euclidean 4,4.

Rendering of the regular 7,3 tiling

Elliptic case

You also asked for the elliptic case, and there the approach is pretty much the same. Use the spherical law of cosines and the surplus angle as a measure of area and you get

\begin{gather*} a = \arccos\frac{\cos\alpha-\cos\beta\cos\gamma}{\sin\beta\sin\gamma} = \arccos\frac{\cos\frac\pi n}{\sin\frac\pi m} \\ A = \frac\pi m+\frac\pi n+\frac\pi 2-\pi = \pi\left(\frac1m+\frac1n-\frac12\right) \\ \frac{(2na)^2}{2nA}=\frac{2n\left(\arccos\frac{\cos\frac\pi n}{\sin\frac\pi m}\right)^2}{\pi\left(\frac1n+\frac1m-\frac12\right)} \end{gather*}

But here the degenerate situations win out. For $m=2$ you get two hemispheres no matter the value of $n$, which is already better than any $m>2$. For $m=1$ you get a division by zero since $\sin\frac\pi m=0$ in that case. But you can well imagine that considering the whole sphere a single cell is optimal as it uses no walls at all.

MvG
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    (+1) Don Hatch's gallery of hyperbolic tessellations is probably of interest. – Andrew D. Hwang Aug 05 '17 at 10:59
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    @AndrewD.Hwang: Thanks! I had considered linking to Don Hatch, or even including an image from there, but in the end I decided that I'd rather spend a bit more time and do my own rendering, since I prefer to have more of a hyperbolic line width towards the rim, to give a better suggestion of the infinity of cells there. Just had to dig out the tools to do so. – MvG Aug 05 '17 at 11:56
  • @MvG: Very nice answer! (+1) Could you provide some info about the tools? – Markus Scheuer Aug 06 '17 at 07:30
  • @Markus: Some Java code I wrote for my Ph.D. There are some bits of it on http://www.morenaments.de/hyp/ but that completely outdated. I should make that available on GitHub, but I won't manage to do so just now. If I do and remember it, I will add a link here. Otherwise if anyone has more than passing interest in that code feel free to contact me by email. – MvG Aug 10 '17 at 18:26
  • @MvG: No, thanks a lot. I was just curious if there are some open SW tools/packages producing such nice graphics. – Markus Scheuer Aug 10 '17 at 18:39