The left is temari (手毬 in Chinese character), Japanese traditional artistic thread balls. Temari's are examples of spherical tilings. The middle is a detail (Euclid or Archimedes) from the fresco The School of Athens by Raphael. The rightmost is Euclid's Elementa Geometriae, the first copy by movable print , 1482, owned by Mathematical Institute, Tohoku University.

# Classification of spherical monohedral polygonal tilings

The following three motivations come from the work of great mathematicians David Hilbert (January 23, 1862 Königsberg - February 14, 1943) and Branko Grünbaum ( 2 October 1929 Croatia - present. A fellow of American mathematical society), as well as from recent study of supramolecules.

## Question No.2 of Hilbert's eighteenth problem adapted for the 2-dimensional sphere.

Hilbert's eighteenth problem, question No.2 asked whether there is an anisohedral polyhedron of euclidean spaces. An anisohedral tile, by definition, admits a monohedral tiling but no isohedral tiling. We say a tiling is isohedral, if the symmetry group acts transitively on the tiles. Karl Reinhardt answered Question No.2 of Hilbert's eighteenth problem in 1928 by finding examples of such polyhedra. Heesch then gave an example of an anisohedral tile in the plane in 1935. By using the terminology of Archimedean duals given in Chapter 21 of (J. H. Conway, H. Burgiel, and C. Goodman-Strauss. The Symmetries of Things. A K Peters Ltd., Wellesley, MA, 2008.), [Sakano-Akama15] classified all spherical monohedral (kite/dart/rhombus)-faced tilings, as follows: The set of spherical monohedral rhombus-faced tilings consists of (1) the central projection of the rhombic dodecahedron, (2) the central projection of the rhombic triacontahedron, (3) a series of non-isohedral tilings, and (4) a series of tilings which are topologically trapezohedra (here a trapezohedron is the dual of an antiprism.). The set of spherical tilings by congruent kites consists of (1) the central projection T of the tetragonal icosikaitetrahedron, (2) the central projection of the tetragonal hexacontahedron, (3) a non-isohedral tiling obtained from T by gliding a hemisphere of T with π/4 radian, and (4) a continuously deformable series of tilings which are topologically trapezohedra. The set of spherical tilings by congruent darts is a continuously deformable series of tilings which are topologically trapezohedra. In the above explanation, unless otherwise stated, the tilings we have enumerated are isohedral and admit no continuous deformation. We prove that if a spherical (kite/dart/rhombus) admits an edge-to-edge spherical monohedral tiling, then it also does a spherical isohedral tiling. We also prove that the set of anisohedral, spherical triangles (i.e., spherical triangles admitting spherical monohedral triangular tilings but not any spherical isohedral triangular tilings) consists of a certain, infinite series of isosceles triangles I, and an infinite series of right scalene triangles which are the bisections of I.

## Skeleton-isohedrality conjecture.

A polytope is said to be regular, if the symmetry group acts transitively on both the vertices and the faces, and semi-regular if the group does transitively on the vertices but not the faces. The regular polytopes are exactly five Platonic solids. The dual of the semi-regular polytopes consist of an infinite series of triangle-faced polytopes (i.e., n-gonal bipyramids. Namely the dual of n-gonal prisms. (n=3,5,6,7,8,...)), an infinite series of quadrangle-faced polytopes (i.e., n-gonal trapezohedrons. Namely, the dual of n-gonal antiprisms. (n=4,5,6,7,8,...)), 7 triangle-faced polytopes, 4 quadrangle-faced polytopes, and 2 pentagon-faced polytopes. The last thirteen semi-regular polytopes are called Archimedean duals. The vertices of some (semi-)regular polytope are not on a single sphere. The skeleton of a spherical tile-transitive (i.e., isohedral) tiling is exactly that of a regular polytope or the dual of a semi-regular polytope, as we see below:

THEOREM([Grünbaum-Shephard81]). For any normal spherical monohedral tiling, if the tiling is isohedral, then the tiling is topologically a Platonic solid, an Archimedean dual, an n-gonal trapezohedron, or an n-gonal bipyramid (n>2).

Here a skeleton is just a graph, which by no means carries information on the edge-lengths and the inner angles. For spherical tilings by congruent polygons, fact the skeleton is that of the duals of a semi-regular polytope does not necessarily imply the isohedrality. Indeed, although a 6-gonal antiprism P is a semi-regular polytope, there exists a spherical tiling A by 12 congruent concave quadrangles such that the skeleton S(A) of A is that of the dual of P, but the tiling A is not isohedral [Akama13].

On the other hand, the tile's convexity is required for a tiling having such a graph to be isohedral, because there is a spherical monohedral tiling A such that A is topologically a trapezohedron of 12 faces (see the graph below), the tile of A is concave (see the image. the tile has three equilateral silver edges and a distinguished golden edge), but A is not isohedral, according to [Akama13]. To see that the tiling A is not isohedral, note that A has only three perpendicular 2-fold rotational symmetries but no mirror planes. The figures from left to right are view from generic viewpoint, view from the antipodal viewpoint, view from a perpendicular 2-fold rotational symmetry axis, view from the north pole, and view from another perpendicular 2-fold rotational symmetry axis.

You can recognize the symmetry through three movies of the spherical tiling where the cameras are moving in following orbits: (1) the equatorial orbit around the 2-fold rotational symmetry axis (movie avi(72MBytes), mp4), (2) a meridian orbit (movie avi(73MBytes), mp4), and (3) the perpendicular meridian orbit (movie avi(75MBytes), mp4). Since the order 4 of the symmetry group is not divisible by the number 12 of tiles, the tiling T is not isohedral although the graph is that of a trapezohedron. Hence, the tile's convexity is required in our conjecture.

Interesting enough, the twelve tiles of the non-isohedral tiling A self-organize A as well as another spherical isohedral tiling (see below) and two tilings have the same graph. This may lead to the self-assembly of supra molecules on the sphere, which is described next.

From viewpoint of classification of spherical monohedral tiling, it is natural to consider a partial inverse of THEOREM: whenever a (semi-)regular polytope other than an atiprism deforms to a spherical tiling T by congruent polygons, T is isohedral. To be more precise,

CONJECTURE. For every spherical tiling T by congruent polygons, for every dual P of a semi-regular polytope, if the skeleton of P is that of T , then T is isohedral or the skeleton of T is that of an n-gonal antiprism for some n. Actually, if the tile of the spherical tiling T is convex, then T is always isohedral.

The conjecture is true for the bipyramids, the seven 3-gon-faced Archimedean duals, and the five Platonic solids, according to [Akama-Yan14] and [Akama-Yan16]. The conjecture is true for the four 4-gon-faced Archimedean duals [Akama16]. In [Akama-Yan16], the combinatorial type of the spherical monohedral polygonal tiling T with the regular dodecahedron skeleton is determined and how T deforms. The degree of freedom of T is two. See the movie and the movie.

## Molecular sphere.

Recently in supramolecular chemistry, self-assembly of shell (equicentric spherical) structures by not only biochemical molecules but also metallic materials are found [MacGillivray12]. A complete catalog of spherical tilings by congruent quadrangles is hopefully useful to analyze the spherical structure of materials newly found or yet to be found. The classification of spherical monohedral polygonal tilings shows the possible forms of molecular sphere[Chand et al.02] of synthetic chemistry. The constructions of spherical monohedral polygonal tilings, some of which are given in [Ueno-Agaoka02] and [Sakano-Akama15], suggest finite, spherical coordination networks which self-organize from small components [Tominaga et al.04] of synthetic chemistry. In particular, each spherical monohedral triangular tiling TI16n+8 and the spherical monohedral kite-faced tiling gDIT admit gliding a hemisphere in a fixed angle. On the other hand, some spherical monohedral triangular tilings and the spherical monohedral quadrangular tilings TRPZ2nα admit continuous deformation. We hope that these discontinuous deformations and continuous deformations are related to the stability of such molecular spheres.

# Reference

1. Yohji Akama, Spherical tilings by congruent 4-gons on Archimedean dual skeletons, Spherical tilings by congruent 4-gons on Archimedean dual skeletons, The 12th Annual Meeting of the International Academy of Mathematical Chemistry (IAMC) and the 2016 International Conference on Mathematical Chemistry (ICMC 2016), Nankai University, June 13, 2016
2. Yohji Akama and Nicolas van Cleemput, Spherical tilings by congruent quadrangles: Forbidden cases and substructures, Ars Mathematica Contemporanea, Vol. 8, No. 2, pp.297-318, 2015.
3. Yohji Akama and Nicolas van Cleemput. Spherical tilings by congruent quadrangles. Combinatorial algorithms and algorithmic graph theory, the Department of Applied Mathematics and Computer science of Ghent University. slides
4. Yohji Akama and Yudai Sakano. Classification of spherical tilings by congruent quadrangles over pseudo-double wheels (III) - the essential uniqueness in case of convex tiles. Submitted, 2013. arXiv:1312.3026[math.MG]
5. Yohji Akama and Kosuke Nakamura. Spherical tilings by congruent quadrangles over pseudo-double wheels (II) - the ambiguity of the inner angles, Submitted, 2013.
6. Yudai Sakano and Yohji Akama. Anisohedral spherical triangles and classification of spherical tilings by congruent kites, darts, and rhombi, Hiroshima mathematical journal 45(3):309-339, 2015. (with Yudai Sakano)
7. Yohji Akama. CLASSIFICATION OF SPHERICAL TILINGS BY CONGRUENT QUADRANGLES(Talk given at Department of Applied Mathematics and Computer Science, on February 9, 2012.)
8. 赤間陽二, 坂野雄大, 中村公亮. 合同な四角形による球面タイリングの分類, 2011年度冬のLAシンポジウム, 平成24年3月19日.
9. Yohji Akama. Classification of spherical tilings by congruent quadrangles over pseudo-double wheels (I) - a special tiling by congruent concave quadrangles. Hiroshima Mathematical Journal, 43(3), 2013, (accepted 12 Dec 2011). arXiv:1210.0152v2 [math.MG]
10. Gunnar Brinkmann, Sam Greenberg, Catherine Greenhill, Brendan D. McKay, Robin Thomas, and Paul Wollan. Generation of simple quadrangulations of the sphere. Discrete Math., 305(1-3):33--54, 2005.
11. Dillip Kumar Chand, Kumar Biradha, Makoto Fujita, Shigeru Sakamoto and Kentaro Yamaguchi. A molecular sphere of octahedral symmetry Chem. Commun., pp. 2486-2487, issue 21, 2002
12. M. Tominaga, K. Suzuki, M. Kawano, T. Kusukawa, T. Ozeki, S. Sakamoto, K. Yamaguchi, and M. Fujita. Finite, spherical coordination networks that self-organize from 36 small components. Angewandte Chemie International Edition, 43(42):5621--5625, 2004.
13. Leonard R. MacGillivray, Design Rules: A Net and Archimedean Polyhedra Score Big for Self-Assembly, Angewandte Chemie International Edition, 51(5):1521-3773, 2012
14. Yukako Ueno and Yoshio Agaoka. Classification of tilings of the 2-dimensional sphere by congruent triangles. Hiroshima Math. J., 32(3):463-540, 2002.
15. Yukako Ueno and Yoshio Agaoka. Examples of spherical tilings by congruent quadrangles. Mem. Fac. Integrated Arts and Sci., Ser. IV, 27:135--144, 2001.
16. B. Grünbaum and G.C. Shephard. Spherical tilings with transitivity properties, in: Davies et al. eds. The geometric vein, Springer, New York, 1981, pp. 65--98.
17. Reinhardt, Karl. Zur Zerlegung der euklidischen Raüme in kongruente Polytope. Sitzungsberichte der Preussischen Akamemie der Wissenschaften Berlin, Physikalisch-Mathematische Klasse: 150--155, 1928.
18. Heesch. H. Aufbau der Ebene aus kongruenten Bereichen. Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse, Neue Folge 1: 115--117. 1935. (transcription by Berglund, with English translation) Retrieved 2007-09-09.
19. R. Diestel. Graph theory, 4th Edition, Vol. 173 of Graduate Texts in Mathematics, Springer, Heidelberg, 2010.
20. Yohji Akama and Min YAN. On Deformed Dodecahedron Tiling. arXiv:1403.6907[math.MG]