Cube-Octahedron Dual

A complex function you can hold.

This sculpture is a picture of a single mathematical function. A rational function on the Riemann sphere is pinned down by its zeros and poles. Here they come from a polyhedron: zeros sit at the 6 vertices of an octahedron, and poles sit at the 8 vertices of its dual, the cube. The surface rises where the function's size grows, so poles become spikes and zeros become pits. The result has 8 spikes pointing to the corners of a cube, and 6 pits facing the corners of an octahedron.

The math

• Function: f(z) = V(z)⁴ / F(z)³. V and F are Klein's classical octahedral forms: V = z(z⁴ − 1) and F = z⁸ + 14z⁴ + 1.

• Zeros of order 4 at the octahedron's vertices, one for each edge meeting there. Poles of order 3 at the cube's vertices, one for each side of the octahedron's triangular faces. The degree is 24.

• The radius at each point of the sphere follows |f|. The constant is fixed so that |f| has geometric mean 1 over the sphere, which puts "sea level" midway between the spikes and the pits.

Files

Two sizes, measured tip to tip: 80 mm and 130 mm.

• Whole model: one piece for each size.

• Cut halves: the model is cut through its centre along a symmetry plane, so the two halves are identical. Print the half file twice. Each half lies flat on its cut face.

• Hanging ornament: halves a and b with a 1.5 mm thread hole through one spike, 9 mm from its tip. Thread it, tie a loop, and hang it on a tree.

Assembly

• Every half has a 5.2 mm alignment hole in the middle of its cut face, for an M5 dowel pin (5.00 mm diameter):

• 80 mm: M5 × 10 pin, 6 mm holes;

• 130 mm: M5 × 20 pin, 11 mm holes.

• With the settings below, the pin gives a light friction fit. Glue the pin into one half, then glue the halves together; the pin keeps them aligned. If your printer makes holes tighter or looser, ream the hole lightly. You can also glue the halves without a pin.

• Glue: a budget cyanoacrylate (super glue) for plastics works well with PLA. I used LOOCTOT glue for plastic.

• Ornament hole: run a 1.5 mm drill bit through it by hand in a pin vise to clean it up. Monofilament (fishing line) makes a good hanging loop. Keep glue away from the hole while assembling.

Print settings (tested)

• PLA, 0.4 mm nozzle

• 0.20 mm layers ("QUALITY" profile)

• 15% gyroid infill

• Halves print cut face down. No supports are needed.

Finishing

Minor stringing can occur between the spikes. I remove it with brief passes of hot air from an SMD rework station. Keep the airflow moving and apply heat sparingly: the thin PLA tips soften quickly. If a spike bends slightly, let it cool enough to touch safely, then gently reshape it by hand while it is still pliable and hold it in position until it sets.

Part of Polyhedra as Complex Functions: how this piece is made, an interactive 3D view of its zeros and poles, and the object's own page. Code and data: GitHub.