Grand antiprism

The grand antiprism or gap, also known as the pentagonal double antiprismoid or decafold pentaantiprismatoswirlchoron, is a convex uniform polychoron that consists of 300 tetrahedra and 20 pentagonal antiprisms. 12 tetrahedra and 2 pentagonal antiprisms join at each vertex. It may be constructed by taking two orthogonal rings of 10 pentagonal antiprisms each, and connecting them by tetrahedra. Alternatively, one may construct the grand antiprism as a faceting of the hexacosichoron, specifically by removing two orthogonal rings of 10 vertices. The resulting diminishings intersect, thus leading to pentagonal antiprisms instead of icosahedra as cells. It is the first in an infinite family of isogonal pentagonal antiprismatic swirlchora.

The grand antiprism is the only convex uniform polychoron where the number of edge types in its highest symmetry is greater than the number of degrees of freedom. In this case, it has three edge types, but only two degrees of freedom.

Despite the name, the grand antiprism is neither a stellation nor an antiprism in any common sense of the word. It is, however, related to the duoantiprisms, being the convex hull of the compound of two orthogonal pentagonal-pentagonal duoantiprisms or two inversely oriented pentagonal antiditetragoltriates formed from pentagons with a size ratio of 1:$$\tfrac{1+\sqrt5}{2}$$. As such, it is also the fourth member of the double antiprismoids and the only convex uniform one, formed from alternating the decagonal ditetragoltriate and then filling the gaps with tetrahedra.

One unusual property of the grand antiprism is that it contains the vertices of a small prismatodecachoron of edge length $$\tfrac{1+\sqrt{5}}{2}$$. This is due to the fact that the inscribed small prismatodecachoron, which has gyrochoric symmetry, can be thought of as a convex hull of two orthogonal stretched 10-3 step prisms.

Its vertex figure is topologically equivalent to the Johnson solid sphenocorona, but with the edge between the two tetragonal faces made longer. This vertex figure can be formed by deleting two adjacent vertices from the regular icosahedron.

Vertex coordinates
The vertices of a grand antiprism of edge length 1 are given by:


 * $$±\left(±\frac{3+\sqrt5}{4},\,0,\,\frac{1+\sqrt5}{4},\,-\frac12\right),$$
 * $$±\left(±\frac12,\,0,\,\frac{3+\sqrt5}{4},\,-\frac{1+\sqrt5}{4}\right),$$
 * $$±\left(0,\,±\frac{3+\sqrt5}{4},\,\frac12,\,\frac{1+\sqrt5}{4}\right),$$
 * $$±\left(0,\,±\frac12,\,\frac{1+\sqrt5}{4},\,\frac{3+\sqrt5}{4}\right),$$
 * $$\left(0,\,0,\,±\frac{1+\sqrt5}{2},\,0\right),$$
 * $$\left(0,\,0,\,0,\,±\frac{1+\sqrt5}{2}\right),$$
 * $$\left(±\frac{1+\sqrt5}{4},\,±\frac{1+\sqrt5}{4},\,±\frac{1+\sqrt5}{4},\,±\frac{1+\sqrt5}{4}\right),$$
 * $$\left(±\frac{1+\sqrt5}{4},\,0,\,±\frac12,\,±\frac{3+\sqrt5}{4}\right),$$
 * $$\left(0,\,±\frac{1+\sqrt5}{4},\,±\frac{3+\sqrt5}{4},\,±\frac12\right),$$
 * $$\left(±\frac{1+\sqrt5}{4},\,±\frac12,\,±\frac{3+\sqrt5}{4},\,0\right),$$
 * $$\left(±\frac{3+\sqrt5}{4},\,±\frac{1+\sqrt5}{4},\,±\frac12,\,0\right),$$
 * $$\left(±\frac12,\,±\frac{3+\sqrt5}{4},\,±\frac{1+\sqrt5}{4},\,0\right),$$
 * $$\left(±\frac{3+\sqrt5}{4},\,±\frac12,\,0,\,±\frac{1+\sqrt5}{4}\right),$$
 * $$\left(±\frac12,\,±\frac{1+\sqrt5}{4},\,0,\,±\frac{3+\sqrt5}{4}\right),$$
 * $$\left(±\frac{1+\sqrt5}{4},\,±\frac{3+\sqrt5}{4},\,0,\,±\frac12\right).$$

These coordinates are formed by removing 20 vertices, in 2 rings of 10, from a regular hexacosichoron.

Variations
The grand antiprism is part of a continuum of more general isogonal variations that can also be called a pentagonal double antiprismoid or a decafold pentaantiprismatoswirlchoron. Both variants have two degrees of freedom and one degree of variation. As a pentagonal double antiprismoid, it has 20 pentagonal antiprisms, 100 tetragonal disphenoids and 200 sphenoids, while maintaining the grand antiprism's symmetry. As a decafold pentaantiprismatoswirlchoron, with half the symmetry of the grand antiprism, its cells are 20 pentagonal gyroprisms, 100 phyllic disphenoids and 200 irregular tetrahedra; its faces are 20 pentagons, 100 isosceles triangles and 200+200+200 scalene triangles and it has 100+100+100+200 edges.