# Octahedron

A solid figure having eight triangular faces, twelve edges and six vertices, with 4 faces at each vertex. If all edges have the same length, it is one of the five regular polyhedra (Platonic solids); if the edge length is , then the volume of the octahedron is

Figure: o068100a

#### Comments

The Schläfli symbol of an octahedron is . When the edges all have the same length one deals with the regular octahedron, reciprocal to the cube; it can be regarded either as a triangular anti-prism or as a square double-pyramid. As one of the five Platonic polyhedra (cf. Regular polyhedra; Platonic solids) it represents the ancient element air. It occurs in nature as a crystal of chrome alum. In terms of its circumradius as unit of measurement, its six vertices have Cartesian coordinates

thus it has edge-length , inradius and volume . Its 4 pairs of opposite faces (or the 4 diameters of the cube) are freely permuted by the octahedral group of order .

#### References

[a1] | G.T. Bennett, "Deformable octahedra" Proc. London Math. Soc. (2) , 10 (1912) pp. 309–343 |

[a2] | H.S.M. Coxeter, "Regular polytopes" , Methuen (1948) pp. 5 |

**How to Cite This Entry:**

Octahedron.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Octahedron&oldid=32562