Bipyramid

A bipyramid is a polyhedron constructed by fusing two pyramids which share the same polygonal base; a pyramid is in turn constructed by connecting each vertex of its base to a single new vertex (the apex) not lying in the plane of the base, for an gonal base forming triangular faces in addition to the base face. An gonal bipyramid thus has faces, edges, and vertices. More generally, a right pyramid is a pyramid where the apices are on the perpendicular line through the centroid of an arbitrary polygon or the incenter of a tangential polygon, depending on the source. Likewise, a right bipyramid is a polyhedron constructed by attaching two symmetrical right bipyramid bases; bipyramids whose apices are not on this line are called oblique bipyramids. When the two pyramids are mirror images, the bipyramid is called symmetric. It is called regular if its base is a regular polygon. When the base is a regular polygon and the apices are on the perpendicular line through its center (a regular right bipyramid) then all of its faces are isosceles triangles; sometimes the name bipyramid refers specifically to symmetric regular right bipyramids, Examples of such bipyramids are the triangular bipyramid, octahedron (square bipyramid) and pentagonal bipyramid. If all their edges are equal in length, these shapes consist of equilateral triangle faces, making them deltahedra; the triangular bipyramid and the pentagonal bipyramid are Johnson solids, and the regular octahedron is a Platonic solid. The symmetric regular right bipyramids have prismatic symmetry, with dihedral symmetry group of order : they are unchanged when rotated of a turn around the axis of symmetry, reflected across any plane passing through both apices and a base vertex or both apices and the center of a base edge, or reflected across the mirror plane. Because their faces are transitive under these symmetry transformations, they are isohedral. They are the dual polyhedra of prisms and the prisms are the dual of bipyramids as well; the bipyramids vertices correspond to the faces of the prism, and the edges between pairs of vertices of one correspond to the edges between pairs of faces of the other, and vice versa. The prisms share the same symmetry as the bipyramids. The regular octahedron is more symmetric still, as its base vertices and apices are indistinguishable and can be exchanged by reflections or rotations; the regular octahedron and its dual, the cube, have octahedral symmetry. The volume of a symmetric bipyramid is \frac{2}{3}Bh, where is the area of the base and the perpendicular distance from the base plane to either apex. In the case of a regular sided polygon with side length and whose altitude is , the volume of such a bipyramid is: \frac{n}{6}hs^2 \cot \frac{\pi}{n}. == Related and other types of bipyramid ==

Concave bipyramids A concave bipyramid has a concave polygon base, and one example is a concave tetragonal bipyramid or an irregular concave octahedron. A bipyramid with an arbitrary polygonal base could be considered a right bipyramid if the apices are on a line perpendicular to the base passing through the base's centroid. Asymmetric bipyramids An asymmetric bipyramid has apices which are not mirrored across the base plane; for a right bipyramid this only happens if each apex is a different distance from the base. The dual of an asymmetric right -gonal bipyramid is an -gonal frustum. A regular asymmetric right -gonal bipyramid has symmetry group , of order . Scalene triangle bipyramids An isotoxal right (symmetric) di--gonal bipyramid is a right (symmetric) -gonal bipyramid with an isotoxal flat polygon base: its basal vertices are coplanar, but alternate in two radii. All its faces are congruent scalene triangles, and it is isohedral. It can be seen as another type of a right symmetric di--gonal scalenohedron, with an isotoxal flat polygon base. An isotoxal right (symmetric) di--gonal bipyramid has two-fold rotation axes through opposite basal vertices, reflection planes through opposite apical edges, an -fold rotation axis through apices, a reflection plane through base, and an -fold rotation-reflection axis through apices, All its faces are congruent scalene triangles, and it is isohedral. It can be seen as another type of a right symmetric di--gonal bipyramid, with a regular zigzag skew polygon base. A regular right symmetric di--gonal scalenohedron has two-fold rotation axes through opposite basal mid-edges, reflection planes through opposite apical edges, an -fold rotation axis through apices, and a -fold rotation-reflection axis through apices (about which rotations-reflections globally preserve the solid), A regular right symmetric star bipyramid has congruent isosceles triangle faces, and is isohedral. A -bipyramid has Coxeter diagram . == 4-polytopes with bipyramidal cells ==

The dual of the rectification of each convex regular 4-polytopes is a cell-transitive 4-polytope with bipyramidal cells. In the following: • is the apex vertex of the bipyramid; • is an equator vertex; • is the distance between adjacent vertices on the equator (equal to 1); • is the apex-to-equator edge length; • is the distance between the apices. The bipyramid 4-polytope will have vertices where the apices of bipyramids meet. It will have vertices where the type vertices of bipyramids meet. • {{tmath|N_\overline{AE} }} bipyramids meet along each type edge. • {{tmath|N_\overline{EE} }} bipyramids meet along each type edge. • {{tmath|C_\overline{AE} }} is the cosine of the dihedral angle along an edge. • {{tmath|C_\overline{EE} }} is the cosine of the dihedral angle along an edge. As cells must fit around an edge, \begin{align} N_\overline{EE} \arccos C_\overline{EE} &\le 2\pi, \\[4pt] N_\overline{AE} \arccos C_\overline{AE} &\le 2\pi. \end{align} ==Other dimensions==

A generalized -dimensional "bipyramid" is any -polytope constructed from an -polytope base lying in a hyperplane, with every base vertex connected by an edge to two apex vertices. If the -polytope is a regular polytope and the apices are equidistant from its center along the line perpendicular to the base hyperplane, it will have identical pyramidal facets. A 2-dimensional analog of a right symmetric bipyramid is formed by joining two congruent isosceles triangles base-to-base to form a rhombus. More generally, a kite is a 2-dimensional analog of a (possibly asymmetric) right bipyramid, and any quadrilateral is a 2-dimensional analog of a general bipyramid. ==See also==

• Chapter 4: Duals of the Archimedean polyhedra, prisms and antiprisms • ==External links==