Realizing (and stellating) deltahedra

Not every triangulation of the sphere can be realized as a deltahedron, that is, a solid having faces that are all equilateral triangles. Furthermore, if such a deltahedron does exist, its trirectangular stellation (which is what knotology weaving produces) may have faces that intersect. The former problem has been investigated in Tsuruta, Naoya, et al. "Enumeration of deltahedral graphs with up to 10 vertices." Proceeding of the 16th International Conference on Geometry and Graphics (ICGG2014). The images shown here have been marked up from that paper.

Basically, from our road code weaving perspective, three things can go wrong with a triangulation on the sphere: it might be multicursal (marked up with an X in the figure above), it might be non-deltahedral (an N in Tsuruta et al.'s code for the candidate deltahedron), or it might be non-stellate-able using trirectangular pyramids (marked up with a strike-through).

The first condition can be easily tested in the abstract graph: the graph is unicursal (in all of its embeddings in the plane) if its spanning tree count is odd.

The second condition is difficult to prove, but Tsuruta et al. have calculated computer models of presumed deltahedra on a small number of vertices that are equilateral to within 1 part in 100,000.

The third condition is sometimes easy if there is a dihedral angle in the deltahedron that is not obtuse enough to accomodate trirectangular pyramids on each of the adjoining faces. A trirectangular pyramid has dihedral angles of 54.74 degrees at its base, so a dihedral angle in the deltahedron less obtuse than 2 x 54.74 = 109.48 degrees cannot accomodate stellation. Such an angle is noticeably more obtuse than 90 degrees, so any dihedral angle of 90-degrees or less is disqualifying on sight.

It is worth noting that Tsuruta et al. use the same Plantri-based naming convention as here, but they count indexes from one rather than zero. Thus the pentagonal dipyramid is 7₄ in their nomenclature, but 7-3 here.

Also Tsuruta et al. exclude deltahedra candidates having dihedral angles of exactly 180 degrees; from a basketry perspective there is not really a need to object to these passages of flat weaving. For example in the figure below, Tsuruta et al. exclude 5 of the 14 candidate deltahedra on 8 vertices (N in their code stands for non-deltahedral) because of they 180-degree dihedral angles. Coincidentally we would exclude all the same (and more) for not being unicursal (marked up with X).

A knotted kagome "nanotube"

Among the smallest knotted (unicursal or single-cycle) kagome baskets are some elongated small-diameter shapes reminiscent of carbon nanotubes. Shown above is the medial graph of Plantri 15-253174, which has the following road code: 10.17 4.21 24.1 27.6 31.12 14.33 8.37 2.41 42.23 43.0 25.44 46.39 48.19 29.50 52.35 54.15 32.55 13.56 57.34 53.58 59.16 11.60 30.61 51.62 63.36 9.64 65.18 49.66 28.67 7.68 69.38 47.70 71.20 5.72 26.73 45.74 75.40 3.76 77.22

This shape can be lengthened 6 vertices at a time. Its shorter predecessor is Plantri 9-12 of the previous post. The endcaps in both baskets are the same, the middle section, which in the 3-regular Tait graph is all hexagonal faces, gets longer 6 hexagons at a time. In carbon chemistry this would be a non-classical fullerene since each end cap has 1 triangular, 1 square, and 1 pentagon face. The face histogram for the 3-regular graph is [0 0 0 2 2 2 9].

Unicursal deltahedra with some symmetry and vertex degrees less than 7

The graph dual of Plantri 9-12, and its realization as knotology weaving

A deltahedron is a polyhedron where all faces are equilateral triangles. A deltahedron is unicursal if its skeletal graph is a Tait graph of a knot. The "knot" in such case is a kagome basket, the weaving of which is easily encoded because of the simple structure of a knot as opposed to a multicomponent link.

A 3-connected triangulation of the sphere might describe a deltahedron if the geometry works for equilateral faces. The Plantri software which is built in to SageMath, can generate 3-connected triangulations. These can be filtered for unicursality by counting spanning trees (another feature built in to Sage Math): a plane graph is unicursal if it has an odd number of spanning trees. In the list of candidate unicursal deltahedra given here, the results have been further filtered to have no vertex of degree 7 or higher, and to have some symmetry, |Aut|>1, as these are perhaps the most interesting to weave.

The "road codes" given will be explained in a later post. The "Plantri identifiers" given here are the number of vertices in the triangulation followed by the listing ordinal when Plantri is asked to generate all 3-connected triangulations on the sphere with that number of vertices.

Unicursal polyhedral triangulations with |Aut|>1 and vertex degrees < 7

Plantri 5-0: |Aut|= 12 Vdeg = [2 3]
7.2 0.9 5.10 12.3 13.8 1.14 6.15 11.16 17.4

Plantri 7-1: |Aut|= 4 Vdeg = [2 3 0 2]
0.9 11.6 3.14 16.1 8.17 19.10 20.5 13.22 2.23 24.15 25.4 21.26 12.27 7.28 29.18

Plantri 7-2: |Aut|= 6 Vdeg = [3 0 3 1]
0.11 12.5 7.14 16.9 3.18 20.1 10.21 22.15 23.8 17.24 2.25 26.19 27.4 13.28 6.29

Plantri 7-3: |Aut|= 20 Vdeg = [0 5 2]
7.0 3.10 13.6 9.16 2.17 19.12 20.5 15.22 8.23 1.24 25.18 26.11 27.4 21.28 14.29

Plantri 8-8: |Aut|= 2 Vdeg = [1 3 3 1]
11.2 4.13 16.9 17.0 6.19 15.22 23.10 24.1 25.18 7.26 21.28 14.29 30.3 31.12 5.32 33.20 34.27 35.8

Plantri 9-12: |Aut|= 2 Vdeg = [2 2 2 3]
4.11 14.1 17.6 8.19 2.23 24.13 25.0 15.26 28.21 30.9 18.31 7.32 33.20 29.34 35.10 5.36 16.37 27.38 39.22 3.40 41.12

Plantri 9-27: |Aut|= 2 Vdeg = [1 4 1 3]
1.8 11.4 2.17 9.18 21.6 24.15 13.26 23.28 29.16 3.30 10.31 19.32 33.0 34.7 35.22 36.27 37.14 25.38 12.39 5.40 20.41

Plantri 9-31: |Aut|= 4 Vdeg = [2 3 0 4]
12.3 5.14 17.8 11.20 21.4 22.13 2.23 25.0 6.27 15.28 30.9 31.18 24.33 34.1 35.26 7.36 16.37 29.38 39.10 40.19 41.32

Plantri 9-32: |Aut|= 2 Vdeg = [2 1 4 2]
4.11 13.2 16.9 7.18 1.22 12.23 24.3 25.14 20.27 30.5 10.31 32.15 33.26 21.34 0.35 29.36 37.6 38.17 8.39 19.40 41.28

Plantri 9-40: |Aut|= 4 Vdeg = [0 5 2 2]
11.2 6.15 18.3 19.12 0.21 9.22 16.25 26.5 28.13 29.20 1.30 10.31 23.32 34.7 14.35 36.27 4.37 38.17 24.39 33.40 41.8

Plantri 9-47: |Aut|= 2 Vdeg = [1 2 5 1]
9.2 7.14 17.4 19.12 21.0 11.24 18.25 5.26 28.15 29.8 30.1 31.22 20.33 13.34 6.35 27.36 37.16 38.3 39.10 40.23 41.32

Plantri 10-140: |Aut|= 2 Vdeg = [1 4 1 4]
0.9 11.2 5.14 21.4 22.13 7.24 17.26 29.20 30.3 31.12 23.32 6.33 15.34 36.27 37.18 39.10 40.1 8.41 25.42 16.43 35.44 45.28 46.19 47.38

Plantri 10-141: |Aut|= 6 Vdeg = [0 3 6 1]
11.2 7.16 19.10 20.1 13.22 5.24 27.18 28.9 14.31 23.32 4.33 35.26 36.17 37.8 29.38 39.0 21.40 12.41 3.42 43.34 44.25 45.6 46.15 30.47

Plantri 10-176: |Aut|= 2 Vdeg = [1 3 3 3]
8.1 6.15 17.4 21.12 23.0 9.24 18.27 5.28 29.16 30.3 32.25 33.10 22.35 13.36 38.19 26.39 40.31 2.41 42.7 14.43 37.44 45.20 46.11 47.34

Plantri 11-382: |Aut|= 2 Vdeg = [2 2 2 5]
13.0 7.20 22.5 24.17 11.26 28.1 29.14 12.31 27.32 33.2 35.16 25.36 10.37 39.4 23.40 41.18 8.43 21.44 45.6 46.19 47.42 9.48 49.38 50.3 51.34 52.15 53.30

Plantri 11-719: |Aut|= 2 Vdeg = [2 2 2 5]
12.1 7.16 21.6 22.15 9.24 27.4 30.13 0.31 11.32 33.2 28.35 5.36 20.37 39.18 41.26 42.3 43.34 29.44 45.14 23.46 8.47 17.48 38.49 50.19 51.40 52.25 53.10

Plantri 11-724: |Aut|= 2 Vdeg = [0 4 4 3]
3.14 17.6 12.21 0.23 24.9 27.20 13.28 2.29 32.7 33.18 4.35 15.36 31.38 39.8 25.40 11.42 43.22 1.44 45.30 46.37 47.16 48.5 34.49 19.50 26.51 41.52 10.53

Plantri 11-739: |Aut|= 2 Vdeg = [0 5 2 4]
1.10 17.0 18.9 6.21 14.23 26.11 27.2 5.30 31.22 15.32 25.34 35.12 37.4 38.29 40.19 8.41 16.43 33.44 24.45 46.13 47.36 48.3 49.28 39.50 51.20 7.52 53.42

Plantri 11-976: |Aut|= 2 Vdeg = [1 4 1 5]
15.2 8.17 20.5 7.22 23.18 27.14 28.1 11.30 24.33 19.34 35.6 36.21 4.37 39.26 40.13 41.0 29.42 10.43 45.32 25.46 38.47 48.3 49.16 9.50 51.44 52.31 53.12

Plantri 11-980: |Aut|= 4 Vdeg = [2 3 0 6]
9.0 6.15 17.4 2.23 25.8 11.28 30.21 19.32 34.13 27.36 10.37 1.38 39.24 40.7 14.41 42.33 43.20 31.44 18.45 5.46 47.16 48.3 22.49 50.29 51.12 35.52 26.53

Plantri 11-1135: |Aut|= 4 Vdeg = [0 5 2 4]
3.10 17.2 18.9 6.21 13.24 27.16 28.1 22.31 32.5 34.19 8.35 37.30 23.38 12.39 41.26 42.15 43.0 29.44 36.45 46.7 20.47 48.33 4.49 11.50 51.40 52.25 53.14

Plantri 11-1155: |Aut|= 2 Vdeg = [3 0 3 5]
11.0 6.13 19.4 2.21 26.7 12.27 1.28 29.22 16.31 33.18 34.3 20.35 5.36 37.14 24.39 41.10 8.43 44.25 38.45 15.46 47.32 48.17 30.49 23.50 51.40 52.9 42.53

Plantri 11-1210: |Aut|= 2 Vdeg = [0 3 6 2]
1.10 15.6 3.20 13.22 25.0 26.9 28.17 4.31 21.32 12.33 35.24 37.8 27.38 39.18 41.30 5.42 14.43 23.44 34.45 46.11 47.2 48.19 49.40 50.29 16.51 7.52 36.53

Plantri 11-1232: |Aut|= 2 Vdeg = [2 1 4 4]
14.3 5.16 18.11 23.2 15.24 4.25 26.13 29.20 31.0 6.33 17.34 35.12 27.36 38.21 39.30 8.41 43.10 19.44 28.45 37.46 47.22 48.1 49.32 7.50 51.42 52.9 40.53

Plantri 12-7571: |Aut|= 2 Vdeg = [1 4 1 6]
15.6 1.20 11.22 24.9 27.18 4.31 12.33 23.34 35.10 36.21 37.2 39.30 5.40 14.41 43.8 25.44 45.0 46.19 47.28 16.49 7.50 42.51 52.13 32.53 54.3 55.38 56.29 57.48 17.58 26.59

Plantri 12-7572: |Aut|= 2 Vdeg = [1 4 1 6]
11.2 5.14 17.8 19.0 7.24 16.25 30.9 31.18 21.34 13.36 4.37 39.28 26.41 42.15 43.6 44.23 33.46 20.47 1.48 10.49 50.29 51.40 27.52 38.53 54.3 55.12 56.35 57.22 45.58 32.59

Plantri 12-7593: |Aut|= 2 Vdeg = [0 3 6 3]
11.0 5.16 20.1 21.12 6.25 17.26 28.3 30.13 31.22 9.34 18.37 27.38 39.4 40.15 42.23 43.32 10.45 35.46 48.7 24.49 50.41 14.51 52.29 2.53 54.19 36.55 47.56 57.8 58.33 59.44

Bumpy Baskets and Geodesics

I participated in the SCULPT Show&Tell earlier this month, with a talk connecting the topic of bumpy baskets to geodesics and, rather slightly, to the interesting topic of closed photon orbits. Thanks to Negar Kalantar and the other organizers for the event and making it available as a YouTube recording. Much cool stuff was presented, my own talk starts at 52 minutes in.

Bumpy Basket files for Cricut

If you want to make your own bumpy basket strips these svg files will be a useful starting point.

A-b training strips, these are about 2 feet long, useful for teaching the basics. These print 7-up on 12" x 24" cardstock.

A-k weavers, these are about 6 feet long, a practical length for beginners to work with. These print 4-up on 12" x 24" cardstock. Each weaver needs two splices using 3/4" Scotch tape 44mm in length, run the tape straight across, wrapping around the back of the strip.

These files were created in Inkscape, and Cricut Design Space does not allow sharing imported files, so you will have to follow this procedure to get the svg files imported into your Cricut Design Space:

In Cricut Design Space:
+New Project
Upload
Upload Image
Drag the svg file into the window
Continue
Upload
If necessary, scale with lock-on to H = 23.386"
Arrange: Ungroup
The character paths should now be be selectable as a group
Change the grouped character paths from Basic Cut to Pen
Select All
Attach
Make

I find the 65-lb cardstock commonly found in craft stores near other Cricut supplies cuts well at the Medium-weight Cardstock setting. Both gel pens and marker-type pens work well, but gel pens need occasional cleaning between prints, while Cricut-brand markers will smear if the cardstock ever gets wet. I use 12" x 24" standard grip Cricut Adhesive Cutting Mats, and you will find it very time efficient to have two. Contrary to folklore, do not need to anchor the corners of the cardstock with blue tape. What you need is the rolling pin in your kitchen drawer. Just lightly roll the the cardstock from every direction. Use more pressure if your mat is losing its grippiness.

More Bumpy Baskets!

Reload or drag downward for a new basket.

There are 3 rules:

“What time is it?”
Each strip is a timeline that you travel along starting at Big A. At each crossing you are meeting with a moment in the past. So, “What time is it now?” at Aa? at Ba? at Hj? Always know the time.

“Big dale!” (said like "Big deal!”)
Hills are easy, saddles are easy, but dales are hard. When the time is big, so are your responsibilities. When the time is big, a completed opening is going to be a dale (you'll find it on your left: completed openings behind you are just saddles) and needs to be forced to bend inward while you are making the crossing.

“Big A rules!”
Big A is the only exception to the 'geology rule' that the present goes on top of the past. Big A has a double, namely the letter in the doubled alphabet that comes right after the last letter in the code. In the final move, Big A Jr. goes directly on top of Big A and makes a dale.