Small stellated hecatonicosachoron

The small stellated hecatonicosachoron, or sishi, also commonly called the small stellated 120-cell, is one of the 10 regular Schläfli–Hess polychora. It has 120 small stellated dodecahedra as cells, joining 3 to an edge and 12 to a vertex in a dodecahedral arrangement.

Vertex coordinates
The vertices of a small stellated hecatonicosachoron of edge length 1, centered at the origin, are given by all permutations of: and all even permutations of:
 * $$\left(\pm1,\,0,\,0,\,0\right)$$,
 * $$\left(\pm\frac12,\,\pm\frac12,\,\pm\frac12,\,\pm\frac12\right)$$,
 * $$\left(\pm\frac{1+\sqrt5}{4},\,\pm\frac{\sqrt5-1}{4},\,\pm\frac12,\,0\right)$$.

The first two sets of vertices form a unit icositetrachoron that can be inscribed into the small stellated hecatonicosachoron. This corresponds to the fact there is a compound of 25 icositetrachora with the same vertices and edges as the small stellated hecatonicosachoron.

Related polychora
The small stellated hecatonicosachoron is the colonel of a regiment containing 26 uniform polychora. Of these, one other is regular (the great grand hecatonicosachoron), three are semiregular (the small ditrigonary hexacosihecatonicosachoron, ditrigonary dishecatonicosachoron, and great ditrigonary hexacosihecatonicosachoron), 3 are noble (the invertidodecahedronary hecatonicosachoron, pentagrammic-prismatic heptacosiicosachoron, and pentagonal-prismatic heptacosiicosachoron), 8 others have full symmetry, and the other 10 (including 4 discovered in 2020) have lower symmetries. The regiment also includes at least 7 uniform compounds, 10 fissary uniforms, and an unknown number (at least several hundred) of scaliforms (all but 2 of the latter have been discovered in 2020 or later).

A significant number of uniform polychora can be edge-inscribed into the small stellated hecatonicosachoron. These include the tesseract, the icositetrachoron, the small prismatodecachoron, the small ditrigonary icosidodecahedral prism, the pentagonal-pentagrammic duoprism, the great duoantiprism, the pentacubic extended duoantiprism, and the snub hexecontatetrasnub-snub disoctachoron.

Uniform [[polychoron compound]s composed of small stellated hecatonicosachora include:


 * Small stellated diacositetracontachoron (2)
 * [[Small stellated chirohexacosichoron (5)
 * Small stellated chirododecahedral hyperprismatochoron (6)
 * Small stellated dishexacosichoron (10)
 * [[Small stellated disdodecahedral hyperprismatochoron (12)