Permutohedron

The nth-order permutohedron (sometimes permutahedron) is a convex (n - 1)-polytope whose vertex coordinates are all possible permutations of (1, 2, ..., n). Although these vectors are n-dimensional, they all fall in a (n - 1)-hyperplane as the sum of the coordinates is constant. Two vertices are connected by an edge if they are related by swapping exactly two coordinates whose values differ by one. Despite the suffix, only the 3-dimensional permutohedron is a polyhedron; a more accurate name would have been "permutotope."

The first few permutohedra are the point (order 1), line segment (order 2), regular hexagon (order 3), truncated octahedron (order 4), great prismatodecachoron (order 5), and great cellidodecateron (order 6). The nth-order permutohedron has n! vertices, (n - 1) n! / 2 edges, and $$(n - d)! \left\{{n \atop n - d}\right\}$$ facets of rank d where the curly braces denote the Stirling numbers of the second kind.

A permutohedron has a single edge length of $$\sqrt{2}$$ and all vertices have a constant distance from the centroid, so all permutohedra are orbiform.

Permutohedra are closely related to the associahedra.