Eberhard's theorem
In mathematics, and more particularly in polyhedral combinatorics, Eberhard's theorem partially characterizes the multisets of polygons that can form the faces of simple convex polyhedra. It states that, for given numbers of triangles, quadrilaterals, pentagons, heptagons, and other polygons other than hexagons, there exists a convex polyhedron with those given numbers of faces of each type (and an unspecified number of hexagonal faces) if and only if those numbers of polygons obey a linear equation derived from Euler's polyhedral formula. The theorem is named after Victor Eberhard, a blind German mathematician, who published it in  1888 in his habilitation thesis and in expanded form in an 1891 book on polyhedra. Definitions and statement. For an arbitrary convex polyhedron, one can define numbers formula_1, formula_2, formula_3, etc., where formula_4 counts the faces of the polyhedron that have exactly formula_5 sides. A three-dimensional convex polyhedron is defined to be simple when every vertex of the polyhedron is incident to exactly three edges. In a simple polygon, every vertex is incident to three angles of faces, and every edge is incident to two sides of faces. Since the numbers of angles and sides of the faces are  given, one can calculate the three numbers formula_6 (the total number of vertices), formula_7 (the total number of edges), and formula_8 (the total number of faces), by summing over all faces and multiplying by an appropriate factor: and Plugging these values into Euler's polyhedral formula formula_12 and clearing denominators leads to the equation which must be satisfied by the face counts of every simple polyhedron. However, this equation is not affected by the value of formula_14 (as its multiplier formula_15 is zero), and, for some choices of the other face counts, changing formula_14 can change whether or not a  polyhedron with those face counts exists. That is, obeying this equation on the face counts is a necessary condition for the existence of a polyhedron, but not a sufficient condition, and a complete characterization of which face counts are realizable would need to take into account the value of formula_14. Eberhard's theorem implies that the equation above is the only necessary condition that does not depend on formula_14. It states that, if an assignment of numbers to formula_19 (omitting formula_14) obeys the equation then there exists a value of formula_14 and a simple convex polyhedron with exactly formula_4 formula_5 -sided faces for all formula_5. Examples. There are three simple Platonic solids, the tetrahedron, cube, and dodecahedron. The tetrahedron has formula_26, the cube has formula_27, and the dodecahedron has formula_28, with all other values of formula_4 being zero. These three assignments of numbers to formula_4 all obey the equation that Eberhard's theorem requires them to obey. The existence of these polyhedra shows that, for these three assignments of numbers to formula_4, there exists a polyhedron with formula_32. The case of the dodecahedron, with formula_28 and all others except formula_14 zero, describes more generally the fullerenes. There is no fullerene  with formula_35 but these graphs are realizable for any other value of formula_14; see for instance, the 26-fullerene graph, with formula_37. There is no simple convex polyhedron with three triangle faces, three pentagon faces, and no other faces. That is, it is impossible to have a simple convex polyhedron with formula_38, and formula_39 for formula_40. However, Eberhard's theorem states that it should be possible to form a simple polyhedron by adding some number of hexagons, and in this case one hexagon suffices: bisecting a cube on a regular hexagon passing through six of its faces produces two copies  of a simple roofless polyhedron with three triangle faces, three pentagon faces, and one hexagon face. That is, setting formula_35 suffices in this case to produce a realizable combination of face counts. Related results. An analogous result to Eberhard's theorem holds for the existence of polyhedra in which all vertices are incident to exactly four edges. In this case the equation derived from Euler's formula is not affected by the number formula_2 of quadrilaterals, and for every assignment to the numbers of faces of other types that obeys this equation it is possible to choose a number of  quadrilaterals that allows a 4-regular polyhedron to be realized. A strengthened version of Eberhard's theorem states that, under the same conditions as the original theorem, there exists a number formula_43 such that all choices of formula_14 that are greater than equal to formula_43 and have the same parity as formula_43 are realizable by simple convex polyhedra. A theorem of David W. Barnette provides a lower bound on the number of hexagons that are needed, whenever the number of faces of order seven or higher is at least three. It states that, in these cases, For polygons with few  pentagons and many high-order faces, this inequality can force the number of hexagons to be arbitrarily large. More strongly, it can be used to find assignments to the numbers of faces for which the required number of hexagons cannot be bounded by any function of the maximum number of sides of a face. Analogues of Eberhard's theorem have also been studied for other systems of faces and face counts than simple convex polyhedra, for instance for toroidal graphs and for tessellations.