# Császár polyhedron

Császár polyhedron
An animation of the Császár polyhedron being rotated and unfolded
Type Toroidal polyhedron
Faces 14 triangles
Edges 21
Vertices 7
χ 0 (Genus 1)
Vertex configuration 3.3.3.3.3.3
Symmetry group C1, [ ]+, (11)
Dual polyhedron Szilassi polyhedron
Properties Nonconvex

In geometry, the Császár polyhedron (Hungarian pronunciation: [ˈtʃaːsaːr]) is a nonconvex polyhedron, topologically a toroid, with 14 triangular faces.

This polyhedron has no diagonals; every pair of vertices is connected by an edge. The seven vertices and 21 edges of the Császár polyhedron form an embedding of the complete graph ${\displaystyle K_{7}}$ onto a topological torus. Of the 35 possible triangles from vertices of the polyhedron, only 14 are faces.

## Complete graph

The tetrahedron and the Császár polyhedron are the only two known polyhedra (having a manifold boundary) without any diagonals: every two vertices of the polygon are connected by an edge, so there is no line segment between two vertices that does not lie on the polyhedron boundary. That is, the vertices and edges of the Császár polyhedron form a complete graph.

If the boundary of a polyhedron with v vertices forms a surface with h holes, in such a way that every pair of vertices is connected by an edge, it follows by some manipulation of the Euler characteristic that

${\displaystyle h={\frac {(v-3)(v-4)}{12}}.}$

This equation is satisfied for the tetrahedron with h = 0 and v = 4, and for the Császár polyhedron with h = 1 and v = 7. The next possible solution, h = 6 and v = 12, would correspond to a polyhedron with 44 faces and 66 edges, but it is not realizable as a polyhedron. It is not known whether such a polyhedron exists with a higher genus (Ziegler 2008).

More generally, this equation can be satisfied only when v is congruent to 0, 3, 4, or 7 modulo 12 (Lutz 2001).

## History and related polyhedra

There are other known polyhedra such as the Schönhardt polyhedron for which there are no interior diagonals (that is, all diagonals are outside the polyhedron) as well as non-manifold surfaces with no diagonals (Szabó 1984, 2009).

The Császár polyhedron is named after Hungarian topologist Ákos Császár, who discovered it in 1949. The dual to the Császár polyhedron, the Szilassi polyhedron, was discovered later, in 1977, by Lajos Szilassi; it has 14 vertices, 21 edges, and seven hexagonal faces, each sharing an edge with every other face. Like the Császár polyhedron, the Szilassi polyhedron has the topology of a torus.

## References

• Császár, A. (1949), "A polyhedron without diagonals", Acta Sci. Math. Szeged, 13: 140–142.
• Gardner, Martin (1988), Time Travel and Other Mathematical Bewilderments, W. H. Freeman and Company, pp. 139–152, ISBN 0-7167-1924-X
• Gardner, Martin (1992), Fractal Music, Hypercards and More: Mathematical Recreations from Scientific American, W. H. Freeman and Company, pp. 118–120, ISBN 0-7167-2188-0
• Lutz, Frank H. (2001), "Császár's Torus", Electronic Geometry Models: 2001.02.069.
• Szabó, Sándor (1984), "Polyhedra without diagonals", Periodica Mathematica Hungarica, 15 (1): 41–49, doi:10.1007/BF02109370.
• Szabó, Sándor (2009), "Polyhedra without diagonals II", Periodica Mathematica Hungarica, 58 (2): 181–187, doi:10.1007/s10998-009-10181-x.
• Ziegler, Günter M. (2008), "Polyhedral Surfaces of High Genus", in Bobenko, A. I.; Schröder, P.; Sullivan, J. M.; Ziegler, G. M., Discrete Differential Geometry, Oberwolfach Seminars, 38, Springer-Verlag, pp. 191–213, ISBN 978-3-7643-8620-7, arXiv:, doi:10.1007/978-3-7643-8621-4_10.