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octadecahedron
In geometry, an octadecahedron (or octakaidecahedron) is a polyhedron with 18 faces. No octadecahedron is regular; hence, the name does not commonly refer to one specific polyhedron.
In chemistry, "''the'' octadecahedron" commonly refers to a specific structure with C2v symmetry, the edge-contracted icosahedron, formed from a regular icosahedron with one edge contracted. It is the shape of the closo-boranate ion [B11H11]2−.
==Convex==
There are 107,854,282,197,058 topologically distinct ''convex'' octadecahedra, excluding mirror images, having at least 11 vertices.〔(Counting polyhedra )〕 (Two polyhedra are "topologically distinct" if they have intrinsically different arrangements of faces and vertices, such that it is impossible to distort one into the other simply by changing the lengths of edges or the angles between edges or faces.)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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