When doing certain things with polyhedra, like applying Conway's operations or developing frame models for 3D printing, it's helpful to know whether all the faces face directly away from the center of the polyhedron. In other words, would a line passing through the center of the polyhedron and the center of each face be perpendicular to that face?
I've looked through terms for characteristics and symmetries of polyhedra such as here and here, and I don't see a term for that characteristic. For brevity, let's call this characteristic Y for purposes of this question.
- regular: too narrow. Archimedean solids appear to be Y but not regular.
- orthohedral: This set of articles defines orthohedral as having "all faces… equidistant from the center." I'm pretty sure this is too narrow as well, because it would exclude an Archimedean truncated tetrahedron.
- uniform: defined as having "regular polygons as faces and is vertex-transitive". This is closer, but again, I'm pretty sure it's too narrow. For example, the pseudorhombicuboctahedron (a Johnson solid) is Y but isn't uniform. A simpler example of a non-uniform Y polyhedron would be a rectangular cuboid.
- Archimedian: If defined as a subset of uniform polyhedra, this is also too narrow. Even if defined more weakly to include pseudo-uniform polyhedra, it excludes most rectangular cuboids.
- Catalan solid: a brief look at these suggests that they may all be Y. (But Catalan solids don't include all Y polyhedra.)
- Johnson solid: Too broad (and too narrow, unless it's defined as including uniform polyhedra). Plenty of Johnson solids, such as J7, are not Y.
- canonical: All edges equidistant from the center. This isn't true for rectangular cuboids.
- circumscribable / equiradial: (Thanks to @OscarLanzi for suggesting this direction.) A polyhedron whose vertices all lie on the surface of a sphere, i.e. equidistant from the sphere's center. But the center of the sphere might not be what you expect as the center of the polyhedron… it might even lie outside the polyhedron.
- Which raises the point that in order to answer this question, one has to first specify what "center" of the polyhedron one has in mind as well as what we mean by the center of each face. Using circumcenter for both might give us a well-defined way to answer this question: Y is equivalent to circumscribable (if that's always true).
- So far, I have been using the arithmetic mean of the vertices as the center of each face and each polyhedron. That seems to work out fine for (true or pseudo) uniform polyhedra. And it has this advantage: For an operation like expansion, in which the faces "are separated and moved radially apart," the center that faces are moving away from would have to be inside the polyhedron. Which isn't always true of the circumcenter.
So, the question is, is there a term for Y (for some reasonable definition of 'center' of face and polyhedron)? If not, is there a term for an equivalent characteristic, i.e. a characteristic that is true for a polyhedron iff Y is true?
Best Answer
As discussed in comments: if a polyhedron is inscribed in a sphere (equiradial), then a normal line to each facial plane passes through the center of the sphere. The appropriate normal to each face would be through the circumcenter of that face (all faces of an equiradial polyhedron are inscribed in circles). Note that the circumcenter of the polygon may lie outside the polygon proper, and the center of the sphere in which the polyhedron is inscribed may likewise lie outside the volume enclosed by the polyhedron.
In such a case, if we define centers as circumcenters or their higher-dimensional equivalents, then all equiradial polyhedra are Y. However, for other polyhedra this type of center is not defined for either the faces (generally) or the polyhedron, so this result cannot be extended to classify other polyhedra.