When Norman Johnson first found, and named, all the Johnson solids in the latter 1960s, he came across a number of “near-misses” — polyhedra which are almost Johnson solids. If you aren’t familiar with the Johnson solids, you can find a definition of them here. The “near-miss” which is most well-known features regular enneagons (nine-sided polygons):
This is the dual of the above polyhedron:
As with all polyhedra and their duals, a compound can be made of these two polyhedra, and here it is:
Finding this polyhedron interesting, I proceeded to use Stella 4d (polyhedron-manipulation software, available at http://www.software3d.com/Stella.php) to make its convex hull.
Here, then, is the dual of this convex hull:
Stella 4d has a “try to make faces regular” function, and I next used it on the polyhedron immediately above. If this function cannot work, though — because making the faces regular is mathematically impossible — one sometimes gets completely unexpected, and interesting, results. Such was the case here.
Next, I found the dual of this latest polyhedron.
The above polyhedron’s “wrinkled” appearance completely surprised me. The next thing I did to change it, once more, was to create this wrinkled polyhedron’s convex hull. A convex hull of a non-convex polyhedron is simply the smallest convex polyhedron which can contain the non-convex polyhedron, and this process often has interesting results.
Next, I created this latest polyhedron’s dual:
I then attempted “try to make faces regular” again, and, once more, had unexpected and interesting results:
The next step was to take the convex hull of this latest polyhedron. In the result, below, all of the faces are kites — two sets of twenty-four each.
I next stellated this kite-faced polyhedron 33 times, looking for an interesting result, and found this:
This looked like a compound to me, so I told Stella 4d to color it as a compound, if possible, and, sure enough, it worked.
The components of this compound looked like triakis tetrahedra to me. The triakis tetrahedron, shown below, is the dual of the truncated tetrahedron. However, I checked the angle measurement of a face, and the components of the above compound-dual are only close, but not quite, to being the same as the true triakis tetrahedron, which is shown below.
This seemed like a logical place to end my latest journey through the world of polyhedra, so I did.
Filed under: Mathematics Tagged: compound, convex hull, dual, enneagon, geometry, journey, mathematics, near-miss, nonagon, polyhedra, polyhedral, polyhedron, stellation, triakis tetrahedron
