# 11-cell

11-cell

The 11 hemi-icosahedra with vertices labeled by indices 0..9,t. Faces are colored by the cell it connects to, defined by the small colored boxes.
Type Abstract regular 4-polytope
Cells 11 hemi-icosahedron
Faces 55 {3}
Edges 55
Vertices 11
Vertex figure hemi-dodecahedron
Schläfli symbol ${\displaystyle \{\{3,5\}_{5},\{5,3\}_{5}\}}$
Symmetry group order 660
Abstract L2(11)
Dual self-dual
Properties Regular

In mathematics, the 11-cell is a self-dual abstract regular 4-polytope (four-dimensional polytope). Its 11 cells are hemi-icosahedral. It has 11 vertices, 55 edges and 55 faces. It has Schläfli type {3,5,3}, with 3 hemi-icosahedra (Schläfli type {3,5}) around each edge.

It has symmetry order 660, computed as the product of the number of cells (11) and the symmetry of each cell (60). The symmetry structure is the abstract group projective special linear group L2(11).

It was discovered in 1977 by Branko Grünbaum, who constructed it by pasting hemi-icosahedra together, three at each edge, until the shape closed up. It was independently discovered by H. S. M. Coxeter in 1984, who studied its structure and symmetry in greater depth.

## Related polytopes

Orthographic projection of 10-simplex with 11 vertices, 55 edges.

The abstract 11-cell contains the same number of vertices and edges as the 10-dimensional 10-simplex, and contains 1/3 of its 165 faces. Thus it can be drawn as a regular figure in 10-space, although then its hemi-icosahedral cells are skew; that is, each cell is not contained within a flat 3-dimensional subspace.