Johnson solid
Convex polyhedron with regular faces
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In geometry, a Johnson solid, sometimes also known as a Johnson–Zalgaller solid,[1] is a convex polyhedron whose faces[2] are regular polygons and that is not a uniform polyhedron.[3][4] There are 92 such solids:
- 48 composed of the primitive pyramids, cupolas, and rotundas assembled in various ways together with prisms and antiprisms;
- 35 formed by modifying uniform polyhedra, by augmenting, diminishing, or gyrating with primitives; and
- 9 which are not derived from "cut-and-paste" manipulations of uniform solids.
Definition and background
A convex polyhedron is the convex hull of a finite set of points in 3-dimensional space, not all in a plane.[5] Its boundary is a finite union of polygons, no two in the same plane; those polygons are called the faces. A Johnson solid is a convex polyhedron[2] whose faces are all regular polygons,[6] but not a uniform polyhedron;[3][4] the last condition excludes the Platonic solids, Archimedean solids, prisms, and antiprisms.
The solids are named after Norman Johnson and Victor Zalgaller.[7] Johnson (1966) published a list of 92 such solids and assigned them their names and numbers. Zalgaller (1969)[8] proved Johnson's conjecture[9] that there were none beyond these 92.
A convex polyhedron in which all faces are nearly regular, but some are not precisely regular, is known as a near-miss Johnson solid.[10]
Naming and construction of solids
- invalid, - Platonic, - Archimedean, - Gyrated sections.
The naming of Johnson solids follows a flexible and precise descriptive formula that allows many solids to be named in multiple different ways without compromising the accuracy of each name as a description. The names of the Johnson solids are described in the following sections.
Pyramids, cupolas, rotundas
The first 48 Johnson solids are constructed from pyramids, cupolas, or rotundas, combined with prisms or antiprisms. The following prefixes are attached to the word to indicate specific combinations of shapes:[11]
- Bi- indicates that two copies of the solid are joined base-to-base.
- For cupolas and rotundas, ortho- indicates that like faces meet.
- For cupolas and rotundas, gyro- indicates that unlike faces meet.
- Elongated indicates a prism is joined to the base of the solid, or between the bases.
- Gyroelongated indicates an antiprism is joined to the base of the solid, or between the bases.
Using this nomenclature, a pentagonal bipyramid is a solid constructed by attaching two bases of pentagonal pyramids. Triangular orthobicupola is constructed by two triangular cupolas along their bases.
- invalid, - Platonic, - Archimedean.
| Fastigium | |
|---|---|
| gyrobi- | 26 Gyrobifastigium  |
Modified uniform polyhedra
The next 35 Johnson solids are constructed by modifying uniform polyhedra such as prisms, Platonic, or Archimedean solids by adding, subtracting, or rotating pyramids or cupolas. The following prefixes are attached to the word to indicate additions, subtractions, or rotations:[11]
- Augmented indicates a pyramid or cupola is added to one or more faces of the solid in question.
- Diminished indicates a pyramid or cupola is removed from one or more faces of the solid in question.
- Gyrate indicates a cupola mounted on or featured in the solid in question is rotated such that different edges match up.
The three operations—augmentation, diminution, and gyration—can be performed multiple times for certain large solids. Bi- & Tri- indicate a double and triple operation respectively. For example, a bigyrate solid has two rotated cupolas, and a tridiminished solid has three removed pyramids or cupolas. In certain large solids, a distinction is made between solids where altered faces are parallel and solids where altered faces are oblique. Para- indicates the former, that the solid in question has altered parallel faces, and meta- the latter, altered oblique faces. For example, a parabiaugmented solid has had two parallel faces augmented, and a metabigyrate solid has had two oblique cupolas gyrated.[11]
Non cut-and-paste
The last 9 Johnson solids have names based on certain polygon complexes from which they are assembled. These names are defined by Johnson with the following nomenclature:[11]
- A lune is a figure of two triangles attached to opposite sides of a square. Prefixes indicating a complex of lunes are:
- Spheno- is a wedgelike complex of two adjacent lunes. Dispheno- indicates two such complexes.
- Hebespheno- is a blunt complex of three adjacent lunes.
- Suffixes indicating a complex of triangles are:
- -corona is a crownlike complex of eight triangles.
- -megacorona is a larger crownlike complex of twelve triangles.
- -cingulum is a belt of twelve triangles.
- Suffix -rotunda indicates a complex of two or three pentagons with triangles between them, bearing a structural resemblance to the pentagonal rotunda.
| Snub polyhedra | |
|---|---|
| 84 Snub disphenoid  |
85 Snub square antiprism  |
| Formed from lunes and triangles | |
| 86 Sphenocorona  |
87 Augmented sphenocorona  |
| 88 Sphenomegacorona  |
89 Hebesphenomegacorona  |
| 90 Disphenocingulum  | |
| Rotundoids | |
| 91 Bilunabirotunda  |
92 Triangular hebesphenorotunda  |
Notable subsets
Deltahedra
5 Johnson solids are deltahedra, with only triangle faces.
Chiral solids
The 5 gyroelongated bicupolas or birotundas are chiral and have distinct left-handed and right-handed forms.
Circumscribable solids
25 of the Johnson solids have vertices that exist on the surface of a sphere. All of them can be seen to be related to a Platonic or Archimedean solid by gyration, diminishment, or dissection.[12]