Pentagonal from maps on surfaces and their symmetries

Klara Stokes

Joint work with Milagros Izquierdo and Marston Conder Table of Contents

1 Background: pentagonal geometries

2 Geometric point-circle configurations on surfaces

3 Point-circle realizations of pentagonal geometries

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Table of Contents

1 Background: pentagonal geometries

2 Geometric point-circle configurations on surfaces

3 Point-circle realizations of pentagonal geometries

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A plane configuration is a system of v points and b straight lines arranged in a plane in such a way that every point is incident with r lines and every line is incident with k points. (Hilbert and Cohn-Vossen, and the Imagination,1932)

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Theorem. (Desargues) In a , two triangles are perspective from a point if and only if they are perspective from a line.

The 10 points and 10 lines in Desargues’ theorem form a configuration with high symmetry - any point could have been taken as the center of perspectivity.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Configurations may exist as combinatorial objects without there being any planar realization.

Let P be a set whose elements are called points.

Let L be a set of subsets of P whose elements are called lines.

Then (P, L) is called an geometry or an .

An (P, L) is a partial linear space if at most one line passes through each pair of points.

A partial linear space is r-regular if there are r lines through each point and k-uniform if there are k points on each line. We say that it has order (r, k).

Regular and uniform partial linear spaces are also called combinatorial configurations. Pentagonal geometries from maps Klara Stokes SIGMAP2014 bb bb bb bb b bb bb b bb b b bb b bb bbb bbb b bb b bbb

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A configuration of The Pappus’ The Desargues’ order (r, k) = (2, 3). configuration has configuration has order (r, k) = (3, 3). order (r, k) = (3, 3).

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Generalising the triangle: projective planes.

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b b b b b Triangle PG(2, 2) - of order 2

Geometric property: any two points are collinear and any two lines meet in one point.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Generalising the square: generalised quadrangles.

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GQ(2, 2) Square The Cremona-Richmond configuration Geometric property: for any point x and line l with x 6∈ l there is exactly one line m such that x ∈ m and m and l meet.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Generalising the polygon: generalised polygons / Tits buildings of rank 2.

A generalised n-gon is a partial linear space such that its bipartite incidence (Levy) graph has diameter (longest distance between any vertices) d = n, and girth (length of shortest cycle) g = 2n.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Generalising the polygon: generalised polygons / Tits buildings of rank 2.

A generalised n-gon is a partial linear space such that its bipartite incidence (Levy) graph has diameter (longest distance between any vertices) d = n, and girth (length of shortest cycle) g = 2n.

Equivalent (geometric) definition: A generalised n-gon is an incidence geometry such that there are no ordinary k-gons for k < n as subgeometry, every pair of elements is contained in an ordinary n-gon, there exists an ordinary (n + 1)-gon.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Theorem. (Feit-Higman, 1964) Generalised n-gons are either thin (i.e. with only two points per line or two lines per point) or n = 3, 4, 6 or 8.

So there are no generalised , nor generalised heptagons!

Is this the only way to generalise the polygon?

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A is a partial linear space such that, for each point p, the points that are not collinear with p are on a unique line.

The Desargues’ configuration also has this property: fixing any center (point) of perspectivity p, the points not collinear with p are the points on the corresponding axis (line) of perspectivity.

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Definition. (Ball-Bamberg-Devillers-S. 2013) A pentagonal geometry is a regular and uniform partial linear space such that, for each point p, the points that are not collinear with x are on a unique line - the opposite line popp.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 The deficiency graph of a partial linear space P is the graph G whose vertices are the points of P and two vertices x and y are connected iff x is not collinear to y in P.

Example. The deficiency graph of the Desargues’ configuration is the .

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A Moore graph of girth 5 is a k-regular graph of girth 5 and diameter 2 (which is maximum).

Moore graphs are extremal graphs: given a diameter they have smallest possible girth.

Hoffman and Singleton proved that Moore graphs of odd girth g only exist if g = 5 and k = 2, 3, 7 and possibly 57.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Theorem. (Ball-Bamberg-Devillers-S. 2013) Pentagonal geometries of order (k, k) are all obtained from Moore graphs by tracing a (combinatorial) line through the neighborhood of each vertex. The Moore graph is the deficiency graph of the pentagonal geometry.

So the study of pentagonal geometries of order (k, k) is equivalent to the study of Moore graphs.

These pentagonal geometries are for k = 2: the ordinary pentagon gives: the ordinary pentagon, for k = 3: the Petersen graph gives: the Desargues’ configuration, for k = 7: the Hoffman-Singleton graph gives: a pentagonal geometry of order (7, 7).

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Theorem. (Ball-Bamberg-Devillers-S. 2013) All pentagonal geometries of order (k, k − 1) are constructed from the pentagonal geometries of order (k, k) through the removal of one point and its opposite line.

Pentagonal geometries constructed through product constructions have disconnected deficiency graph. The known pentagonal geometries with connected deficiency graph are of order (r, 2), (3, 3), (7, 6), (7, 7), and (13, 3).

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Table of Contents

1 Background: pentagonal geometries

2 Geometric point-circle configurations on surfaces

3 Point-circle realizations of pentagonal geometries

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A map is a drawing of a graph on a surface, such that the complement of the drawing is the disjoint union of finitely many topological discs called faces.

The genus of a map is the genus of the surface in which the graph is embedded and can be calculated using Eulers formula.

A map is uniform if there are p, q such that each is a q-gon and p faces meet at each vertex, and regular if its automorphism group acts transitively on triples of incident vertices, edges and faces (flags). So regular implies uniform but the opposite is not true.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 G´evay and Pisanski constructed spherical point-circle configurations coming from certain convex polytopes [G´evay-Pisanski, 2014].

Stereographic projection then gives point-circle configurations in the Euclidean plane.

They showed that all Platonic solids except the octahedron gives spherical point-circle configurations.

Only one of these is also a combinatorial point-line configuration: the (203, 203)-configuration constructed from the dodecahedron.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A uniform map on a surface is the quotient of a tiling of congruent polygons in a universal covering space U in congruent polygons by the action of a torsion-free subgroup of a triangle group G ⊆ Γ(p, 2, q).

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A uniform map on a surface is the quotient of a tiling of congruent polygons in a universal covering space U in congruent polygons by the action of a torsion-free subgroup of a triangle group G ⊆ Γ(p, 2, q). Since the polygons are congruent, there are isometric circles through the neighbors of each vertex of the tiling.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A uniform map on a surface is the quotient of a tiling of congruent polygons in a universal covering space U in congruent polygons by the action of a torsion-free subgroup of a triangle group G ⊆ Γ(p, 2, q). Since the polygons are congruent, there are isometric circles through the neighbors of each vertex of the tiling. The action of G maps these circles to isometric circles through the neighbors of each vertex of the uniform map in U/G.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A uniform map on a surface is the quotient of a tiling of congruent polygons in a universal covering space U in congruent polygons by the action of a torsion-free subgroup of a triangle group G ⊆ Γ(p, 2, q). Since the polygons are congruent, there are isometric circles through the neighbors of each vertex of the tiling. The action of G maps these circles to isometric circles through the neighbors of each vertex of the uniform map in U/G. Each circle contains p points and each point is on q circles, so this construction gives a configuration of points and circles on the surface!

Pentagonal geometries from maps Klara Stokes SIGMAP2014 A uniform map on a surface is the quotient of a tiling of congruent polygons in a universal covering space U in congruent polygons by the action of a torsion-free subgroup of a triangle group G ⊆ Γ(p, 2, q). Since the polygons are congruent, there are isometric circles through the neighbors of each vertex of the tiling. The action of G maps these circles to isometric circles through the neighbors of each vertex of the uniform map in U/G. Each circle contains p points and each point is on q circles, so this construction gives a configuration of points and circles on the surface!

Theorem. (Izquierdo-S.) A uniform map on a surface yields a configuration of points and isometric circles on the same surface.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Table of Contents

1 Background: pentagonal geometries

2 Geometric point-circle configurations on surfaces

3 Point-circle realizations of pentagonal geometries

Pentagonal geometries from maps Klara Stokes SIGMAP2014 The Petersen graph and the Desargues’ configuration (order (3, 3)).

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Desargues’ configuration can be drawn with crossings (outside points of the configuration) in any projective plane over a field with enough elements.

It can be drawn without crossings on a non-orientable surface of Euler characteristic -5 [Coxeter, 1975], as a map of the collinearity graph (the Menger graph). Every geodesic contains three edges forming a line of the configuration, so this is not barely a representation of Desargues’ configuration, this IS Desargues’ configuration. This map realizes the entire automorphism group of Desargues’ configuration: The symmetric group on 5 Figure from [Coxeter, 1975]. elements S5 (of order 120).

Pentagonal geometries from maps Klara Stokes SIGMAP2014 It is well-known that there is a map of the Petersen graph in the projective plane as the hemi-dodecahedron, obtained by identifying antipodal points in the dodecahedron on the sphere.

This is a “pentagonal embedding”: the polyhedron consists of 10 vertices, 15 edges and 6 pentagonal faces. It is a tessellation of the projective plane into 6 pentagonal faces.

For each vertex, the position of its three neighbors determine a circle. 4 c 3 bb d b b Let these circles be the “lines” of a 5 bb 2 combinatorial configuration on the e bbb bbb a bb bb same point set as the graph. 1 1

a b b e Result: point-circle realization of bb bb 2 5 Desargues’ configuration in projective bbb bbb b b d plane! 3 c 4

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Pentagonal geometries from maps Klara Stokes SIGMAP2014 Hoffman-Singleton graph and the pentagonal geometry of order (7, 7).

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Hoffman-Singleton graph: Moore graph of girth 5, 50 vertices and 175 edges, automorphism group of graph is PSU(3, 5).2 of order 252 000, largest possible characteristic (smallest genus) will be realized by a pentagonal embedding with 70 faces. Then Euler characteristic is |V | − |E| + |F | = −55: non-orientable.

Lemma. (Conder-S.) There is no regular embedding of the Hoffman-Singleton graph.

Pentagonal geometries from maps Klara Stokes SIGMAP2014 Theorem. (Conder-S.) There are uniform but non-regular pentagonal embedding of the Hoffman-Singleton graph in a surface of genus 57 with trivial automorphism group, 5-folded symmetry, and 7-folded symmetry.

Next slide: Pentagonal embedding of the Hoffman-Singleton graph in a surface of genus 57 with 5-folded symmetry, represented in the Poincar´edisk. Numbers represent identifications of vertices. Vertex 50 is the vertex at the arrow and the rest are numbered in order.

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bb bb 23 43 bb bb 28 41 46 6 8 48 31 21 18 31 47 7 4 3 2 28 19 36 21 26 13 12 Pentagonal geometries from maps Klara Stokes SIGMAP2014 Now draw circles through the neighborhood of each vertex in the map.

Theorem. The pentagonal geometry of order (7, 7) can be realized as a point-circle configuration on a surface of genus 57 with trivial automorphism group, 5-folded symmetry, or 7-folded symmetry.

Corollary. The pentagonal geometry of order (7, 6) can also be realized as a point-circle configuration on a surface of genus 57.

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b 43 b 28 41 46 6 8 48 31 21 18 31 47 7 4 3 2 28 19 36 21 26 13 12 Pentagonal geometries from maps Klara Stokes SIGMAP2014 Thank you for listening!

Pentagonal geometries from maps Klara Stokes SIGMAP2014 S. Ball, J. Bamberg, A. Devillers and K. Stokes. An alternative way to generalise the pentagon, Journal of Combinatorial Designs, 21:4, pp. 163179 (2013). M. Conder and K. Stokes. Minimum genus embeddings of the Homan-Singleton graph. Manuscript. H.S.M. Coxeter. Desargues configurations and their collineation groups. Math. Proc. Camb. Phil. Soc.(1975), 78, 227. G. G´evay and T. Pisanski. Kronecker covers, V-construction, unit-distance graphs and isometric point-circle configurations, ARS Math. Cont. 7 (2014) 317–336. D. Hilbert and S. Cohn-Vossen. Geometry and the imagination, (Orig. Anschauliche Geometrie, Springer, Berlin, 1932), trans. by P. Nemenyi, AMS Chelsey Pub. 1999. K. Stokes and M. Izquierdo. Geometric pentagonal geometries from Moore graphs. Manuscript.

Pentagonal geometries from maps Klara Stokes SIGMAP2014