Pentagonal rotunda

Pentagonal rotunda
(3D model) (OFF file)
Rank	3
Type	CRF
Space	Spherical
Notation
Bowers style acronym	Pero
Coxeter diagram	ofx5xox&#xt
Elements
Faces	5+5 triangles, 1+5 pentagons, 1 decagon
Edges	5+5+5+10+10
Vertices	5+5+10
Vertex figures	5+5 rectangles, edge lengths 1 and 1+√5)/2
	10 scalene triangles, edge lengths 1, (1+√5)/2, √(5+√5)/2
Measures (edge length 1)
Circumradius	$\frac{1+\sqrt5}{2} ≈ 1.61803$
Volume	$\frac{45+17\sqrt5}{12} ≈ 6.91776$
Dihedral angles	3–5: $\arccos\left(-\sqrt{\frac{5+2\sqrt5}{15}}\right) ≈ 142.62263°$
	3–10: $\arccos\left(\sqrt{\frac{5-2\sqrt5}{15}}\right) ≈ 79.18768°$
	5–10: $\arccos\left(\frac{\sqrt5}{5}\right) ≈ 63.43495°$
Central density	1
Related polytopes
Army	Pero
Regiment	Pero
Dual	Semibisected pentagonal rhombitrapezohedron
Conjugate	Pentagrammic rotunda
Abstract & topological properties
Euler characteristic	2
Surface	Sphere
Orientable	Yes
Genus	0
Properties
Symmetry	H2×I, order 10
Convex	Yes
Nature	Tame

The pentagonal rotunda, or pero, is one of the 92 Johnson solids (J₆). It consists of 5+5 triangles, 1+5 pentagons, and 1 decagon. It is a rotunda based on a pentagon, and the only rotunda that results in a Johnson solid.

It can be constructed by cutting an icosidodecahedron in half along one of its decagonal circles of edges. This produces two pentagonal rotundas with the bases in opposite orientation, so the icosidodecahedron can be thought of as the pentagonal gyrobirotunda.

Vertex coordinates

Coordinates for a pentagonal rotunda of edge length 1 has vertices are given by the following:

• $\left(0,\,\sqrt{\frac{5+\sqrt5}{10}},\,\sqrt{\frac{5+2\sqrt5}{5}}\right),$
• $\left(±\frac12,\,-\sqrt{\frac{5+2\sqrt5}{20}},\,\sqrt{\frac{5+2\sqrt5}{5}}\right),$
• $\left(±\frac{1+\sqrt5}{4},\,\sqrt{\frac{5-\sqrt5}{40}},\,\sqrt{\frac{5+2\sqrt5}{5}}\right),$
• $\left(0,\,-\sqrt{\frac{5+2\sqrt5}{5}},\,\sqrt{\frac{5+\sqrt5}{10}}\right),$
• $\left(±\frac{1+\sqrt5}{4},\,\sqrt{\frac{25+11\sqrt5}{40}},\,\sqrt{\frac{5+\sqrt5}{10}}\right),$
• $\left(±\frac{3+\sqrt5}{4},\,-\sqrt{\frac{5+\sqrt5}{40}},\,\sqrt{\frac{5+\sqrt5}{10}}\right),$
• $\left(±\frac12,\,±\frac{\sqrt{5+2\sqrt5}}{2},\,0\right),$
• $\left(±\frac{3+\sqrt5}{4},\,±\sqrt{\frac{5+\sqrt5}{8}},\,0\right),$
• $\left(±\frac{1+\sqrt5}{2},\,0,\,0\right).$

These coordinates create a pentagonal rotunda with a decagonal base on the xy plane.

An alternate set of coordinates can be obtained as a subset of the vertices of the icosidodecahedron:

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

Related polyhedra

Two pentagonal rotundas can be attached at their decagonal bases in the same orientation to form a pentagonal orthobirotunda. If the second rotunda is rotated by 36º the result is the pentagonal gyrobirotunda, better known as the icosidodecahedron. If a pentagonal cupola is attached, the result is either a pentagonal orthocupolarotunda (if the base pentagons are in the same orientation) or a pentagonal gyrocupolarotunda (if the base pentagons are rotated 36º).

A decagonal prism can be attached to the pentagonal rotunda's decagonal base to form the elongated pentagonal rotunda. If a decagonal antiprism is attached instead, the result is the gyroelongated pentagonal rotunda.

External links

• Bowers, Jonathan. "Batch 3: Id, Did, and Gid Facetings" (#4 under id).

• Klitzing, Richard. "pero".

• Quickfur. "The Pentagonal Rotunda".

• Weisstein, Eric W. "Pentagonal Rotunda" ("Johnson solid") at MathWorld.

• Wikipedia Contributors. "Pentagonal rotunda".
• McCooey, David. "Pentagonal Rotunda"