thumb|3D model of a hebesphenomegacorona
In geometry, the hebesphenomegacorona is a Johnson solid with 18 equilateral triangles and 3 squares as its faces.
Properties
The hebesphenomegacorona is named by in which he used the prefix hebespheno- referring to a blunt wedge-like complex formed by three adjacent lunes—a square with equilateral triangles attached on its opposite sides. The suffix -megacorona refers to a crownlike complex of 12 triangles. By joining both complexes together, the result polyhedron has 18 equilateral triangles and 3 squares, making 21 faces. All of its faces are regular polygons, categorizing the hebesphenomegacorona as a Johnson solid—a convex polyhedron in which all of its faces are regular polygons—enumerated as 89th Johnson solid <math> J_{89} </math>. It is an elementary polyhedron, meaning it cannot be separated by a plane into two small regular-faced polyhedra.
The surface area of a hebesphenomegacorona with edge length <math> a </math> can be determined by adding the area of its faces, 18 equilateral triangles and 3 squares
<math display="block"> \frac{6 + 9\sqrt{3{2}a^2 \approx 10.7942a^2, </math>
and its volume is <math> 2.9129a^3 </math>.
Cartesian coordinates
Let <math> a \approx 0.21684 </math> be the second smallest positive root of the polynomial
<math display="block"> \begin{align} &26880x^{10} + 35328x^9 - 25600x^8 - 39680x^7 + 6112x^6 \\ &\quad {}+ 13696x^5 + 2128x^4 - 1808x^3 - 1119x^2 + 494x - 47 \end{align}</math>
Then, Cartesian coordinates of a hebesphenomegacorona with edge length 2 are given by the union of the orbits of the points
<math display="block"> \begin{align} &\left(1,1,2\sqrt{1-a^2}\right),\ \left(1+2a,1,0\right),\ \left(0,1+\sqrt{2}\sqrt{\frac{2a-1}{a-1,-\frac{2a^2+a-1}{\sqrt{1-a^2\right),\ \left(1,0,-\sqrt{3-4a^2}\right), \\ &\left(0,\frac{\sqrt{2(3-4a^2)(1-2a)}+\sqrt{1+a{2(1-a)\sqrt{1+a,\frac{(2a-1)\sqrt{3-4a^2{2(1-a)}-\frac{\sqrt{2(1-2a){2(1-a)\sqrt{1+a\right) \end{align}</math>
under the action of the group generated by reflections about the xz-plane and the yz-plane.