Parallelohedron
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geometry Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...
, a parallelohedron or Fedorov polyhedron is a
convex polyhedron In geometry, a polyhedron (: polyhedra or polyhedrons; ) is a three-dimensional figure with flat polygonal faces, straight edges and sharp corners or vertices. The term "polyhedron" may refer either to a solid figure or to its boundary su ...
that can be translated without rotations to fill
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are ''Euclidean spaces ...
, producing a
honeycomb A honeycomb is a mass of Triangular prismatic honeycomb#Hexagonal prismatic honeycomb, hexagonal prismatic cells built from beeswax by honey bees in their beehive, nests to contain their brood (eggs, larvae, and pupae) and stores of honey and pol ...
in which all copies of the polyhedron meet face-to-face.
Evgraf Fedorov Evgraf Stepanovich Fedorov (, – 21 May 1919) was a Russian mathematician, crystallographer and mineralogist. Fedorov was born in the Russian city of Orenburg. His father was a topographical engineer. The family later moved to Saint Petersb ...
identified the five types of parallelohedron in 1885 in his studies of crystallographic systems. They are the
cube A cube or regular hexahedron is a three-dimensional space, three-dimensional solid object in geometry, which is bounded by six congruent square (geometry), square faces, a type of polyhedron. It has twelve congruent edges and eight vertices. It i ...
,
hexagonal prism In geometry, the hexagonal prism is a Prism (geometry), prism with hexagonal base. Prisms are polyhedrons; this polyhedron has 8 face (geometry), faces, 18 Edge (geometry), edges, and 12 vertex (geometry), vertices.. As a semiregular polyhedro ...
,
rhombic dodecahedron In geometry, the rhombic dodecahedron is a Polyhedron#Convex_polyhedra, convex polyhedron with 12 congruence (geometry), congruent rhombus, rhombic face (geometry), faces. It has 24 edge (geometry), edges, and 14 vertex (geometry), vertices of 2 ...
,
elongated dodecahedron In geometry, the elongated dodecahedron, extended rhombic dodecahedron, rhombo-hexagonal dodecahedron or hexarhombic dodecahedron is a convex dodecahedron with 8 Rhombus, rhombic and 4 hexagonal faces. The hexagons can be made equilateral, or re ...
, and
truncated octahedron In geometry, the truncated octahedron is the Archimedean solid that arises from a regular octahedron by removing six pyramids, one at each of the octahedron's vertices. The truncated octahedron has 14 faces (8 regular hexagon, hexagons and 6 Squa ...
. Each parallelohedron is centrally symmetric with symmetric faces, making it a special case of a
zonohedron In geometry, a zonohedron is a convex polyhedron that is point symmetry, centrally symmetric, every face of which is a polygon that is centrally symmetric (a zonogon). Any zonohedron may equivalently be described as the Minkowski addition, Minkows ...
. Each parallelohedron is also a
stereohedron In geometry and crystallography, a stereohedron is a convex polyhedron that isohedral tiling, fills space isohedrally, meaning that the symmetries of the tiling take any copy of the stereohedron to any other copy. Two-dimensional analogues to the ...
, a polyhedron that tiles space so that all tiles are symmetric. The centers of the tiles in a tiling of space by parallelohedra form a
Bravais lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 ...
, and every Bravais lattice can be formed in this way. Adjusting the lengths of parallel edges in a parallelohedron, or performing an
affine transformation In Euclidean geometry, an affine transformation or affinity (from the Latin, '' affinis'', "connected with") is a geometric transformation that preserves lines and parallelism, but not necessarily Euclidean distances and angles. More general ...
of the parallelohedron, results in another parallelohedron of the same combinatorial type. It is possible to choose this adjustment so that the tiling by parallelohedra is the
Voronoi diagram In mathematics, a Voronoi diagram is a partition of a plane into regions close to each of a given set of objects. It can be classified also as a tessellation. In the simplest case, these objects are just finitely many points in the plane (calle ...
of its Bravais lattice, and so that the resulting parallelohedra become special cases of the plesiohedra. The three-dimensional parallelohedra are analogous to two-dimensional
parallelogon In geometry, a parallelogon is a polygon with Parallel (geometry), parallel opposite sides (hence the name) that can Tessellation, tile a Plane (geometry), plane by Translation (geometry), translation (Rotation (mathematics), rotation is not per ...
s and higher-dimensional parallelotopes.


Definition and construction

A parallelohedron is defined to be a polyhedron whose translated copies meet face-to-face to fill space, forming a
honeycomb A honeycomb is a mass of Triangular prismatic honeycomb#Hexagonal prismatic honeycomb, hexagonal prismatic cells built from beeswax by honey bees in their beehive, nests to contain their brood (eggs, larvae, and pupae) and stores of honey and pol ...
. The resulting honeycomb must be periodic, having a three-dimensional system of global symmetries, because each translation from a copy of the polyhedron to an adjoining copy must apply to all copies, forming a symmetry of the whole tiling. For the same reason, the honeycomb is uniquely determined by the position of any one parallelohedron in it. In order to meet face-to-face with another copy, each face of the polyhedron must correspond to a parallel face with the same shape but the opposite orientation. By a result of
Hermann Minkowski Hermann Minkowski (22 June 1864 – 12 January 1909) was a mathematician and professor at the University of Königsberg, the University of Zürich, and the University of Göttingen, described variously as German, Polish, Lithuanian-German, o ...
, the shape of a parallelohedron is uniquely determined by the
normal vector In geometry, a normal is an object (e.g. a line, ray, or vector) that is perpendicular to a given object. For example, the normal line to a plane curve at a given point is the infinite straight line perpendicular to the tangent line to the cu ...
s and areas of these opposite face pairs. This implies that a parallelohedron must be centrally symmetric, because otherwise a point reflection of the polyhedron would produce a different shape with the same normal vectors and face areas, contradicting Minkowski's uniqueness theorem. Each face of a parallelohedron must also be centrally symmetric, to match its symmetric copy in the adjoining copy of the parallelohedron. These two properties of parallelohedra, having central symmetry and centrally symmetric faces, characterize a broader class of polyhedra, the
zonohedra In geometry, a zonohedron is a convex polyhedron that is point symmetry, centrally symmetric, every face of which is a polygon that is centrally symmetric (a zonogon). Any zonohedron may equivalently be described as the Minkowski addition, Minkows ...
, so every parallelohedron is a zonohedron. In any zonohedron, the edges can be grouped into ''zones'', systems of parallel edges of equal length. If one edge is selected from each zone, the
Minkowski sum In geometry, the Minkowski sum of two sets of position vectors ''A'' and ''B'' in Euclidean space is formed by adding each vector in ''A'' to each vector in ''B'': A + B = \ The Minkowski difference (also ''Minkowski subtraction'', ''Minkowsk ...
of the selected edges gives a translated copy of the zonohedron itself. All Minkowski sums of finite sets of line segments produce zonohedra; the segments forming a zonohedron in this way are called its ''generators''. Unlike some other zonohedra, the parallelohedra can only have from three to six zones and, correspondingly, from three to six generators. Any zonohedron whose faces have the same combinatorial structure as one of the five parallelohedron is itself a parallelohedron. Any
affine transformation In Euclidean geometry, an affine transformation or affinity (from the Latin, '' affinis'', "connected with") is a geometric transformation that preserves lines and parallelism, but not necessarily Euclidean distances and angles. More general ...
of a parallelohedron will produce another parallelohedron of the same type. One way to characterize the parallelohedra among all zonohedra is using ''belts''. define a belt of a zonohedron to be the cycle of faces that contain all parallel copies of one edge. The number of faces in any belt of any zonohedron must be even, and can be any even number greater than two. As Federov and many others showed, the parallelohedra are exactly the zonohedra all of whose belts consist of only four or six faces.


Classification


By combinatorial structure

The five types of parallelohedron, and their most symmetric forms, are as follows. *A
parallelepiped In geometry, a parallelepiped is a three-dimensional figure formed by six parallelograms (the term ''rhomboid'' is also sometimes used with this meaning). By analogy, it relates to a parallelogram just as a cube relates to a square. Three equiva ...
is generated from three line segments that are not all parallel to a common plane. Its most symmetric form is the
cube A cube or regular hexahedron is a three-dimensional space, three-dimensional solid object in geometry, which is bounded by six congruent square (geometry), square faces, a type of polyhedron. It has twelve congruent edges and eight vertices. It i ...
, generated by three perpendicular unit-length line segments. The tiling of space by the cube is the
cubic honeycomb The cubic honeycomb or cubic cellulation is the only proper regular space-filling tessellation (or honeycomb (geometry), honeycomb) in Euclidean 3-space made up of cube, cubic cells. It has 4 cubes around every edge, and 8 cubes around each verte ...
. *A
hexagonal prism In geometry, the hexagonal prism is a Prism (geometry), prism with hexagonal base. Prisms are polyhedrons; this polyhedron has 8 face (geometry), faces, 18 Edge (geometry), edges, and 12 vertex (geometry), vertices.. As a semiregular polyhedro ...
is generated from four line segments, three of them parallel to a common plane and the fourth not. Its most symmetric form is the right prism over a regular hexagon. It tiles space to form the
hexagonal prismatic honeycomb The triangular prismatic honeycomb or triangular prismatic cellulation is a space-filling tessellation (or honeycomb) in Euclidean 3-space. It is composed entirely of triangular prisms. It is constructed from a triangular tiling extruded into pr ...
. *The
rhombic dodecahedron In geometry, the rhombic dodecahedron is a Polyhedron#Convex_polyhedra, convex polyhedron with 12 congruence (geometry), congruent rhombus, rhombic face (geometry), faces. It has 24 edge (geometry), edges, and 14 vertex (geometry), vertices of 2 ...
is generated from four line segments, no two of which are parallel to a common plane. Its most symmetric form is generated by the four long diagonals of a cube. It tiles space to form the rhombic dodecahedral honeycomb. The
Bilinski dodecahedron In geometry, the Bilinski dodecahedron is a Convex set, convex polyhedron with twelve Congruence (geometry), congruent golden rhombus faces. It has the same topology as the face-transitive rhombic dodecahedron, but a different geometry. It is a ...
, another less-symmetric form of the rhombic dodecahedron, is notable for (like the symmetric rhombic dodecahedron) having all of its faces congruent; its faces are golden rhombi. *The
elongated dodecahedron In geometry, the elongated dodecahedron, extended rhombic dodecahedron, rhombo-hexagonal dodecahedron or hexarhombic dodecahedron is a convex dodecahedron with 8 Rhombus, rhombic and 4 hexagonal faces. The hexagons can be made equilateral, or re ...
is generated from five line segments, with two triples of coplanar segments. It can be generated by using an edge of the cube and its four long diagonals as generators. *The
truncated octahedron In geometry, the truncated octahedron is the Archimedean solid that arises from a regular octahedron by removing six pyramids, one at each of the octahedron's vertices. The truncated octahedron has 14 faces (8 regular hexagon, hexagons and 6 Squa ...
is generated from six line segments with four triples of coplanar segments. It can be embedded in four-dimensional space as the 4- permutahedron, whose vertices are all permutations of the counting numbers (1,2,3,4). In three-dimensional space, its most symmetric form is generated from six line segments parallel to the face diagonals of a cube. The tiling of space generated by its translations has been called the
bitruncated cubic honeycomb The bitruncated cubic honeycomb is a space-filling tessellation (or honeycomb (geometry), honeycomb) in Euclidean 3-space made up of truncated octahedron, truncated octahedra (or, equivalently, Bitruncation (geometry), bitruncated cubes). It has 4 ...
.


By symmetries and Bravais lattices

The lengths of the segments within each zone can be adjusted arbitrarily, independently of the other zones. Doing so extends or shrinks the corresponding edges of the parallelohedron, without changing its combinatorial type or its property of tiling space. As a limiting case, for a parallelohedron with more than three parallel classes of edges, the length of one of these classes can be adjusted to zero, producing a different parallelohedron of a simpler form, with one fewer zone. Beyond the central symmetry common to all zonohedra and all parallelohedra, additional symmetries are possible with an appropriate choice of the generating segments. When further subdivided according to their symmetry groups, there are 22 forms of the parallelohedra. For each form, the centers of its copies in its honeycomb form the points of one of the 14
Bravais lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 ...
s. Because there are fewer Bravais lattices than symmetric forms of parallelohedra, certain pairs of parallelohedra map to the same Bravais lattice. By placing one endpoint of each generating line segment of a parallelohedron at the origin of three-dimensional space, the generators may be represented as three-dimensional vectors, the positions of their opposite endpoints. For this placement of the segments, one vertex of the parallelohedron will itself be at the origin, and the rest will be at positions given by sums of certain subsets of these vectors. A parallelohedron with g vectors can in this way be parameterized by 3g coordinates, three for each vector, but only some of these combinations are valid (because of the requirement that certain triples of segments lie in parallel planes, or equivalently that certain triples of vectors are coplanar) and different combinations may lead to parallelohedra that differ only by a rotation, scaling transformation, or more generally by an
affine transformation In Euclidean geometry, an affine transformation or affinity (from the Latin, '' affinis'', "connected with") is a geometric transformation that preserves lines and parallelism, but not necessarily Euclidean distances and angles. More general ...
. When affine transformations are factored out, the number of free parameters that describe the shape of a parallelohedron is zero for a parallelepiped (all parallelepipeds are equivalent to each other under affine transformations), two for a hexagonal prism, three for a rhombic dodecahedron, four for an elongated dodecahedron, and five for a truncated octahedron.


History

The classification of parallelohedra into five types was first made by Russian crystallographer
Evgraf Fedorov Evgraf Stepanovich Fedorov (, – 21 May 1919) was a Russian mathematician, crystallographer and mineralogist. Fedorov was born in the Russian city of Orenburg. His father was a topographical engineer. The family later moved to Saint Petersb ...
, as chapter 13 of a Russian-language book first published in 1885, whose title has been translated into English as ''An Introduction to the Theory of Figures''. Federov was working under an incorrect theory of the structure of
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s, according to which every crystal has a repeating structure in the shape of a parallelohedron, which in turn is formed from one or more
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s that all take the same shape (a
stereohedron In geometry and crystallography, a stereohedron is a convex polyhedron that isohedral tiling, fills space isohedrally, meaning that the symmetries of the tiling take any copy of the stereohedron to any other copy. Two-dimensional analogues to the ...
). This theory was falsified by the 1913 discovery of the structure of
halite Halite ( ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pi ...
(table salt) which is not partitioned into separate molecules, and more strongly by the much later discovery of
quasicrystal A quasiperiodicity, quasiperiodic crystal, or quasicrystal, is a structure that is Order and disorder (physics), ordered but not Bravais lattice, periodic. A quasicrystalline pattern can continuously fill all available space, but it lacks trans ...
s. Some of the mathematics in Federov's book is faulty; for instance it includes an incorrect proof of a lemma stating that every monohedral tiling of the plane is periodic, proven to be false in 2023 as part of the solution to the . In the case of parallelohedra, Fedorov assumed without proof that every parallelohedron is centrally symmetric, and used this assumption to prove his classification. The classification of parallelohedra was later placed on a firmer footing by
Hermann Minkowski Hermann Minkowski (22 June 1864 – 12 January 1909) was a mathematician and professor at the University of Königsberg, the University of Zürich, and the University of Göttingen, described variously as German, Polish, Lithuanian-German, o ...
, who used his uniqueness theorem for polyhedra with given face normals and areas to prove that parallelohedra are centrally symmetric.


Related shapes

In two dimensions the analogous figure to a parallelohedron is a
parallelogon In geometry, a parallelogon is a polygon with Parallel (geometry), parallel opposite sides (hence the name) that can Tessellation, tile a Plane (geometry), plane by Translation (geometry), translation (Rotation (mathematics), rotation is not per ...
, a polygon that can tile the plane edge-to-edge by translation. There are two kinds of parallelogons: the
parallelogram In Euclidean geometry, a parallelogram is a simple polygon, simple (non-list of self-intersecting polygons, self-intersecting) quadrilateral with two pairs of Parallel (geometry), parallel sides. The opposite or facing sides of a parallelogram a ...
s and the
hexagon In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is de ...
s in which each pair of opposite sides is parallel and of equal length. There are multiple non-convex polyhedra that tile space by translation, beyond the five Federov parallelohedra. These are not zonohedra and need not be centrally symmetric. For instance, some of these can be obtained from a
rhombic triacontahedron The rhombic triacontahedron, sometimes simply called the triacontahedron as it is the most common thirty-faced polyhedron, is a convex polyhedron with 30 rhombus, rhombic face (geometry), faces. It has 60 edge (geometry), edges and 32 vertex ...
by replacing certain triples of faces by indentations. According to a conjecture of
Branko Grünbaum Branko Grünbaum (; 2 October 1929 – 14 September 2018) was a Croatian-born mathematician of Jewish descentconvex polytope A convex polytope is a special case of a polytope, having the additional property that it is also a convex set contained in the n-dimensional Euclidean space \mathbb^n. Most texts. use the term "polytope" for a bounded convex polytope, and the wo ...
that tiles space by translation is called a ''parallelotope''. There are 52 different four-dimensional parallelotopes, first enumerated by
Boris Delaunay Boris Nikolayevich Delaunay or Delone (; 15 March 1890 – 17 July 1980) was a Soviet and Russian mathematician, mountain climber, and the father of physicist, Nikolai Borisovich Delone. He is best known for the Delaunay triangulation. Biograph ...
(with one missing parallelotope, later discovered by Mikhail Shtogrin), and exactly 110,244 types in five dimensions. Unlike the case for three dimensions, not all of them are
zonotope In geometry, a zonohedron is a convex polyhedron that is point symmetry, centrally symmetric, every face of which is a polygon that is centrally symmetric (a zonogon). Any zonohedron may equivalently be described as the Minkowski addition, Minkows ...
s. 17 of the four-dimensional parallelotopes are zonotopes, one is the regular
24-cell In four-dimensional space, four-dimensional geometry, the 24-cell is the convex regular 4-polytope (four-dimensional analogue of a Platonic solid) with Schläfli symbol . It is also called C24, or the icositetrachoron, octaplex (short for "octa ...
, and the remaining 34 of these shapes are
Minkowski sum In geometry, the Minkowski sum of two sets of position vectors ''A'' and ''B'' in Euclidean space is formed by adding each vector in ''A'' to each vector in ''B'': A + B = \ The Minkowski difference (also ''Minkowski subtraction'', ''Minkowsk ...
s of zonotopes with the 24-cell. A d-dimensional parallelotope can have at most 2^-2 facets, with the
permutohedron In mathematics, the permutohedron (also spelled permutahedron) of order is an -dimensional polytope embedded in an -dimensional space. Its vertex (geometry), vertex coordinates (labels) are the permutations of the first natural numbers. The edg ...
achieving this maximum. Every parallelohedron is a
stereohedron In geometry and crystallography, a stereohedron is a convex polyhedron that isohedral tiling, fills space isohedrally, meaning that the symmetries of the tiling take any copy of the stereohedron to any other copy. Two-dimensional analogues to the ...
, a convex polyhedron that tiles space in such a way that there exist symmetries of the tiling that take any tile to any other tile. A
plesiohedron In geometry, a plesiohedron is a special kind of space-filling polyhedron, defined as the Voronoi cell of a symmetric Delone set. Three-dimensional Euclidean space can be completely filled by copies of any one of these shapes, with no overlaps. The ...
is a related class of three-dimensional space-filling polyhedra, formed from the
Voronoi diagram In mathematics, a Voronoi diagram is a partition of a plane into regions close to each of a given set of objects. It can be classified also as a tessellation. In the simplest case, these objects are just finitely many points in the plane (calle ...
s of periodic sets of points (of a more general type than the lattices). The Voronoi diagram of a lattice produces a tiling of space by parallelohedra, but not every parallelohedron and its tiling can be generated in this way: for a parallelohedron to be a plesiohedron, it is required that each vector from the center of the parallelohedron to the center of a face be perpendicular to the face. However, as
Boris Delaunay Boris Nikolayevich Delaunay or Delone (; 15 March 1890 – 17 July 1980) was a Soviet and Russian mathematician, mountain climber, and the father of physicist, Nikolai Borisovich Delone. He is best known for the Delaunay triangulation. Biograph ...
proved in 1929, every parallelohedron can be made into a plesiohedron by an affine transformation. He also proved the same fact for four-dimensional parallelohedra, and it has been proven as well for five dimensions, but this remains open in higher dimensions. Some other three-dimensional plesiohedra are not parallelohedra. The tilings of space by plesiohedra have symmetries taking any cell to any other cell, but unlike for the parallelohedra, these symmetries may involve rotations, not just translations.


See also

* Keller's conjecture, on tilings by translated copies of cubes and hypercubes that are not required to be face-to-face


References


External links

* {{mathworld , title = Parallelohedron , urlname = Parallelohedron Space-filling polyhedra