Snub (geometry)

In geometry, a snub is an operation applied to a polyhedron.

The term originates from Kepler's names of two Archimedean solids, for the snub cube (cubus simus) and snub dodecahedron (dodecaedron simum).

[1] In general, snubs have chiral symmetry with two forms: with clockwise or counterclockwise orientation.

By Kepler's names, a snub can be seen as an expansion of a regular polyhedron: moving the faces apart, twisting them about their centers, adding new polygons centered on the original vertices, and adding pairs of triangles fitting between the original edges.

The terminology was generalized by Coxeter, with a slightly different definition, for a wider set of uniform polytopes.

John Conway explored generalized polyhedron operators, defining what is now called Conway polyhedron notation, which can be applied to polyhedra and tilings.

Conway calls Coxeter's operation a semi-snub.

[2] In this notation, snub is defined by the dual and gyro operators, as s = dg, and it is equivalent to an alternation of a truncation of an ambo operator.

Conway's notation itself avoids Coxeter's alternation (half) operation since it only applies for polyhedra with only even-sided faces.

In 4-dimensions, Conway suggests the snub 24-cell should be called a semi-snub 24-cell because, unlike 3-dimensional snub polyhedra are alternated omnitruncated forms, it is not an alternated omnitruncated 24-cell.

This definition is used in the naming of two Johnson solids: the snub disphenoid and the snub square antiprism, and of higher dimensional polytopes, such as the 4-dimensional snub 24-cell, with extended Schläfli symbol s{3,4,3}, and Coxeter diagram .

A regular polyhedron (or tiling), with Schläfli symbol

, and Coxeter diagram , has truncation defined as

, and , and has snub defined as an alternated truncation

A quasiregular polyhedron, with Schläfli symbol

or r{p,q}, and Coxeter diagram or , has quasiregular truncation defined as

or tr{p,q}, and or , and has quasiregular snub defined as an alternated truncated rectification

For example, Kepler's snub cube is derived from the quasiregular cuboctahedron, with a vertical Schläfli symbol

, and Coxeter diagram , and so is more explicitly called a snub cuboctahedron, expressed by a vertical Schläfli symbol

Regular polyhedra with even-order vertices can also be snubbed as alternated truncations, like the snub octahedron, as

The snub octahedron represents the pseudoicosahedron, a regular icosahedron with pyritohedral symmetry.

, and , is the alternation of the truncated tetrahedral symmetry form,

Coxeter's snub operation also allows n-antiprisms to be defined as

is a regular n-hosohedron, a degenerate polyhedron, but a valid tiling on the sphere with digon or lune-shaped faces.

The same process applies for snub tilings: Nonuniform polyhedra with all even-valance vertices can be snubbed, including some infinite sets; for example: Snub star-polyhedra are constructed by their Schwarz triangle (p q r), with rational ordered mirror-angles, and all mirrors active and alternated.

In general, a regular polychoron with Schläfli symbol

, and Coxeter diagram , has a snub with extended Schläfli symbol

, and Coxeter diagram , and the snub 24-cell is represented by

It also has an index 6 lower symmetry constructions as

or sr{3,3,4,3} and or , and lowest symmetry form as