Regular, SemiRegular, Regular faced and Archimedean polytopes
Definition
- A polytope is the convex hull of a finite set of points in the d-dimensional space R^d.
- a flag is a sequence F0, ..., Fd of faces of P with Fi included in Fi+1.
- Regular polytopes: the symmetry group of P is transitive on the set of flags of P.
- Regular-faced d-polytope: the facets are regular polytopes.
- Semi-regular d-polytope: regular-faced d-polytope with equivalent vertices (i.e. the group of symmetries of the polytope is transitive on vertices)
- Archimedean 4-polytopes: facets are regular or archimedean 3-polytopes.
Classification of regular polytopes
This classification was obtained in
H.S.M. Coxeter, "Regular polytopes" New York 1963.
Classification of semi-regular polytopes
- All regular polytopes
- 0_21 also called hypersimplex
- 1_21, half-5-cube
- 2_21, Delaunay polytope of the root lattice E6
- 3_21, Delaunay polytope of the root lattice E7
- 4_21, Voronoi polytope of the root lattice E8
- snub 24-cell
- octicosahedric polytope, i.e. the medial of 600-cell.
This classification was obtained in
G.Blind and R.Blind, "The semi-regular polyhedra", Commentari Mathematici Helvetici 66 (1991) 150--154.
T.Gosset, "On the regular and semiregular figures in spaces of $n$ dimensions", Messenger of Mathematics 29 (1900) 43--48.
Classification of regular-faced polytopes
This classification was obtained in
G.Blind and R.Blind, "Die Konvexen Polytope im $\R^4$, bei denen alle Facetten regulare Tetraeder sind", Monatshefte fur Mathematik 89 (1980) 87--93.
Classification of Archimedean 4-polytopes
- 45 polytopes obtained by Wythoff's kaleidoscope construction from 4-dimensional irreducible reflection (point) groups;
- 17 prisms on Platonic (other than Cube) and Archimedean solids;
- prisms on the Antiprism;
- a doubly infinite set of polytopes, which are direct products of two regular polygons (if one of polygons is a square, then we get prisms on 3-dimensional prisms);
- the snub 24-cell;(explained also in Regular polytopes )
- a new polytope, called Grand Antiprism See below a short desciption of how to construct it
- The 600-cell is a regular polytope having 600 simplicial facets belonging to 120 vertices.
- The set of vertex adjacent to a given vertex form an isosahedron. So, given a vertex v and another vertex w adjacent to v, there exist an unique opposite vertex w' of w relatively to v
- There exist a circuit v1, ..., v10 of vertices with v_{i+1} being opposite to v_{i-1} relatively to v_{i}
- Given one such circuit, the antipodal make another such circuit.
- Take one circuit and its antipodal and remove it from the list of vertices of 600-cell.
- The obtained polytope has 100 vertices, its facet set consist of 300 simplices and 20 5-antiprisms. This polytope is vertex transitive.
This classification is announced in
J.H.Conway, "Four-dimensional Archimedean polytopes", Proc. Colloquium on Convexity, Copenhagen 1965, Kobenhavns Univ. Mat. Institut (1967) 38--39.