Conway polyhedron notation

Source: Wikipedia, the free encyclopedia.
This example chart shows how 11 new forms can be derived from the cube using 3 operations. The new polyhedra are shown as maps on the surface of the cube so the topological changes are more apparent. Vertices are marked in all forms with circles.

In geometry and topology, Conway polyhedron notation, invented by John Horton Conway and promoted by George W. Hart, is used to describe polyhedra based on a seed polyhedron modified by various prefix operations.[1][2]

Conway and Hart extended the idea of using operators, like

Kepler, to build related polyhedra of the same symmetry. For example, tC represents a truncated cube, and taC, parsed as t(aC), is (topologically) a truncated cuboctahedron. The simplest operator dual swaps vertex and face elements; e.g., a dual cube is an octahedron: dC = O. Applied in a series, these operators allow many higher order polyhedra to be generated. Conway defined the operators a (ambo), b (bevel), d (dual), e (expand), g (gyro), j (join), k (kis), m (meta), o (ortho), s (snub), and t (truncate), while Hart added r (reflect) and p (propellor).[3] Later implementations named further operators, sometimes referred to as "extended" operators.[4][5] Conway's basic operations are sufficient to generate the Archimedean and Catalan solids from the Platonic solids
. Some basic operations can be made as composites of others: for instance, ambo applied twice is the expand operation (aa = e), while a truncation after ambo produces bevel (ta = b).

Polyhedra can be studied topologically, in terms of how their vertices, edges, and faces connect together, or geometrically, in terms of the placement of those elements in space. Different implementations of these operators may create polyhedra that are geometrically different but topologically equivalent. These topologically equivalent polyhedra can be thought of as one of many

canonical form
to avoid ambiguity.

Operators

In Conway's notation, operations on polyhedra are applied like functions, from right to left. For example, a cuboctahedron is an ambo cube,[6] i.e. , and a truncated cuboctahedron is . Repeated application of an operator can be denoted with an exponent: j2 = o. In general, Conway operators are not

commutative
.

Individual operators can be visualized in terms of

chiral operators are also called local symmetry-preserving operations (LSP) and local operations that preserve orientation-preserving symmetries (LOPSP), respectively.[7][8][9]
LSPs should be understood as local operations that preserve symmetry, not operations that preserve local symmetry. Again, these are symmetries in a topological sense, not a geometric sense: the exact angles and edge lengths may differ.

Fundamental domains of faces with sides
3 (Triangle) 4 (Square) 5 (Pentagon) 6 (Hexagon)
The fundamental domains for polyhedron groups. The groups are  for achiral polyhedra, and  for chiral polyhedra.

Hart introduced the reflection operator r, that gives the mirror image of the polyhedron.[6] This is not strictly a LOPSP, since it does not preserve orientation: it reverses it, by exchanging white and red chambers. r has no effect on achiral polyhedra aside from orientation, and rr = S returns the original polyhedron. An overline can be used to indicate the other chiral form of an operator: s = rsr.

An operation is irreducible if it cannot be expressed as a composition of operators aside from d and r. The majority of Conway's original operators are irreducible: the exceptions are e, b, o, and m.

Matrix representation

x
xd
dx
dxd

The relationship between the number of vertices, edges, and faces of the seed and the polyhedron created by the operations listed in this article can be expressed as a matrix . When x is the operator, are the vertices, edges, and faces of the seed (respectively), and are the vertices, edges, and faces of the result, then

.

The matrix for the composition of two operators is just the product of the matrixes for the two operators. Distinct operators may have the same matrix, for example, p and l. The edge count of the result is an integer multiple d of that of the seed: this is called the inflation rate, or the edge factor.[7]

The simplest operators, the

identity operator S and the dual operator
d, have simple matrix forms:

,

Two dual operators cancel out; dd = S, and the square of is the identity matrix. When applied to other operators, the dual operator corresponds to horizontal and vertical reflections of the matrix. Operators can be grouped into groups of four (or fewer if some forms are the same) by identifying the operators x, xd (operator of dual), dx (dual of operator), and dxd (conjugate of operator). In this article, only the matrix for x is given, since the others are simple reflections.

Number of operators

The number of LSPs for each inflation rate is starting with inflation rate 1. However, not all LSPs necessarily produce a polyhedron whose edges and vertices form a 3-connected graph, and as a consequence of Steinitz's theorem do not necessarily produce a convex polyhedron from a convex seed. The number of 3-connected LSPs for each inflation rate is .[8]

Original operations

Strictly, seed (S), needle (n), and zip (z) were not included by Conway, but they are related to original Conway operations by duality so are included here.

From here on, operations are visualized on cube seeds, drawn on the surface of that cube. Blue faces cross edges of the seed, and pink faces lie over vertices of the seed. There is some flexibility in the exact placement of vertices, especially with chiral operators.

Original Conway operators
Edge factor Matrix x xd dx dxd Notes
1
Seed: S

Dual: d

Seed: dd = S
Dual replaces each face with a vertex, and each vertex with a face.
2
Join: j

Ambo: a
Join creates quadrilateral faces. Ambo creates degree-4 vertices, and is also called rectification, or the medial graph in graph theory.[10]
3
Kis: k

Needle: n

Zip: z

Truncate: t
Kis raises a pyramid on each face, and is also called akisation,
augmentation. Truncate cuts off the polyhedron at its vertices but leaves a portion of the original edges.[12] Zip is also called bitruncation
.
4
Ortho: o = jj

Expand: e = aa
5
Gyro: g
gd = rgr sd = rsr
Snub: s
Chiral operators. See Snub (geometry). Contrary to Hart,[3] gd is not the same as g: it is its chiral pair.[13]
6
Meta: m = kj

Bevel: b = ta

Seeds

Any polyhedron can serve as a seed, as long as the operations can be executed on it. Common seeds have been assigned a letter. The

anticupolae (Vn); and pyramids (Yn). Any Johnson solid
can be referenced as Jn, for n=1..92.

All of the five Platonic solids can be generated from prismatic generators with zero to two operators:[14]

  • Triangular pyramid
    : Y3 (A tetrahedron is a special pyramid)
    • T = Y3
    • O = aT (ambo tetrahedron)
    • C = jT (join tetrahedron)
    • I = sT (snub tetrahedron)
    • D = gT (gyro tetrahedron)
  • Triangular antiprism: A3 (An octahedron is a special antiprism)
    • O = A3
    • C = dA3
  • Square prism: P4 (A cube is a special prism)
    • C = P4
  • Pentagonal antiprism: A5

The regular Euclidean tilings can also be used as seeds:

Extended operations

These are operations created after Conway's original set. Note that many more operations exist than have been named; just because an operation is not here does not mean it does not exist (or is not an LSP or LOPSP). To simplify, only irreducible operators are included in this list: others can be created by composing operators together.

Irreducible extended operators
Edge factor Matrix x xd dx dxd Notes
4
Chamfer: c

cd = du

dc = ud

Subdivide: u
Chamfer is the join-form of l. See Chamfer (geometry).
5
Propeller: p

dp = pd

dpd = p
Chiral operators. The propeller operator was developed by George Hart.[15]
5
Loft: l

ld

dl

dld
6
Quinto: q

qd

dq

dqd
6
Join-lace: L0

L0d

dL0

dL0d
See below for explanation of join notation.
7
Lace: L

Ld

dL

dLd
7
Stake: K

Kd

dK

dKd
7
Whirl: w
wd = dv
vd = dw
Volute: v Chiral operators.
8
Join-kis-kis:



Sometimes named J.[4] See below for explanation of join notation. The non-join-form, kk, is not irreducible.
10
Cross: X

Xd

dX

dXd

Indexed extended operations

A number of operators can be grouped together by some criteria, or have their behavior modified by an index.[4] These are written as an operator with a subscript: xn.

Augmentation

Augmentation
operations retain original edges. They may be applied to any independent subset of faces, or may be converted into a join-form by removing the original edges. Conway notation supports an optional index to these operators: 0 for the join-form, or 3 or higher for how many sides affected faces have. For example, k4Y4=O: taking a square-based pyramid and gluing another pyramid to the square base gives an octahedron.

Operator k l L K (kk)
x
x0
k0 = j

l0 = c

L0

K0 = jk

Augmentation Pyramid Prism Antiprism

The truncate operator t also has an index form tn, indicating that only vertices of a certain degree are truncated. It is equivalent to dknd.

Some of the extended operators can be created in special cases with kn and tn operators. For example, a

chamfered cube, cC, can be constructed as t4daC, as a rhombic dodecahedron, daC or jC, with its degree-4 vertices truncated. A lofted cube, lC is the same as t4kC. A quinto-dodecahedron, qD can be constructed as t5daaD or t5deD or t5oD, a deltoidal hexecontahedron
, deD or oD, with its degree-5 vertices truncated.

Meta/Bevel

Meta adds vertices at the center and along the edges, while bevel adds faces at the center, seed vertices, and along the edges. The index is how many vertices or faces are added along the edges. Meta (in its non-indexed form) is also called

cantitruncation or omnitruncation. Note that 0 here does not mean the same as for augmentation operations: it means zero vertices (or faces) are added along the edges.[4]

Meta/Bevel operators
n Edge factor Matrix x xd dx dxd
0 3
k = m0

n

z = b0

t
1 6
m = m1 = kj

b = b1 = ta
2 9
m2

m2d

b2

b2d
3 12
m3
m3d b3 b3d
n 3n+3 mn mnd bn bnd

Medial

Medial is like meta, except it does not add edges from the center to each seed vertex. The index 1 form is identical to Conway's ortho and expand operators: expand is also called cantellation and expansion. Note that o and e have their own indexed forms, described below. Also note that some implementations start indexing at 0 instead of 1.[4]

Medial operators
n Edge
factor
Matrix x xd dx dxd
1 4
M1 = o = jj

e = aa
2 7
Medial: M = M2

Md

dM

dMd
n 3n+1 Mn Mnd dMn dMnd

Goldberg-Coxeter

The Goldberg-Coxeter (GC) Conway operators are two infinite families of operators that are an extension of the

for formulas.

The two families are the triangular GC family, ca,b and ua,b, and the quadrilateral GC family, ea,b and oa,b. Both the GC families are indexed by two integers and . They possess many nice qualities:

  • The indexes of the families have a relationship with certain
    Gaussian integers
    for the quadrilateral GC family.
  • Operators in the x and dxd columns within the same family commute with each other.

The operators are divided into three classes (examples are written in terms of c but apply to all 4 operators):

  • Class I: . Achiral, preserves original edges. Can be written with the zero index suppressed, e.g. ca,0 = ca.
  • Class II: . Also achiral. Can be decomposed as ca,a = cac1,1
  • Class III: All other operators. These are chiral, and ca,b and cb,a are the chiral pairs of each other.

Of the original Conway operations, the only ones that do not fall into the GC family are g and s (gyro and snub). Meta and bevel (m and b) can be expressed in terms of one operator from the triangular family and one from the quadrilateral family.

Triangular

Triangular Goldberg-Coxeter operators
a b Class Edge factor
T = a2 + ab + b2
Matrix Master triangle x xd dx dxd
1 0 I 1
u1 = S

d

c1 = S
2 0 I 4
u2 = u

dc

du

c2 = c
3 0 I 9
u3 = nn

nk

zt

c3 = zz
4 0 I 16
u4 = uu
uud = dcc duu = ccd c4 = cc
5 0 I 25
u5
u5d = dc5 du5 = c5d c5
6 0 I 36
u6 = unn
unk czt u6 = czz
7 0 I 49
u7 = u2,1u1,2 = vrv
vrvd = dwrw dvrv = wrwd c7 = c2,1c1,2 = wrw
8 0 I 64
u8 = u3
u3d = dc3 du3 = c3d c8 = c3
9 0 I 81
u9 = n4
n3k = kz3 tn3 = z3t c9 = z4
1 1 II 3
u1,1 = n

k

t

c1,1 = z
2 1 III 7 v = u2,1
vd = dw
dv = wd
w = c2,1
3 1 III 13 u3,1 u3,1d = dc3,1 du3,1 = c3,1d
c3,1
3 2 III 19 u3,2 u3,2d = dc3,2 du3,2 = c3,2d
c3,2
4 3 III 37 u4,3 u4,3d = dc4,3 du4,3 = c4,3d
c4,3
5 4 III 61 u5,4 u5,4d = dc5,4 du5,4 = c5,4d
c5,4
6 5 III 91 u6,5 = u1,2u1,3 u6,5d = dc6,5 du6,5 = c6,5d
c6,5=c1,2c1,3
7 6 III 127 u7,6 u7,6d = dc7,6 du7,6 = c7,6d
c7,6
8 7 III 169 u8,7 = u3,12 u8,7d = dc8,7 du8,7 = c8,7d
c8,7 = c3,12
9 8 III 217 u9,8 = u2,1u5,1 u9,8d = dc9,8 du9,8 = c9,8d
c9,8 = c2,1c5,1
I, II, or III ... ua,b ua,bd = dca,b dua,b = ca,bd ca,b
I or III ... ua,b ua,bd = dca,b dua,b = ca,bd ca,b

By basic number theory, for any values of a and b, .

Quadrilateral

Quadrilateral Goldberg-Coxeter operators
a b Class Edge factor
T = a2 + b2
Matrix Master square x xd dx dxd
1 0 I 1
o1 = S

e1 = d

o1 = dd = S
2 0 I 4
o2 = o = j2

e2 = e = a2
3 0 I 9
o3

e3

o3
4 0 I 16
o4 = oo = j4

e4 = ee = a4
5 0 I 25
o5 = o2,1o1,2 = prp
e5 = e2,1e1,2
o5= dprpd
6 0 I 36
o6 = o2o3
e6 = e2e3
7 0 I 49
o7
e7
o7
8 0 I 64
o8 = o3 = j6
e8 = e3 = a6
9 0 I 81
o9 = o32

e9 = e32

o9
10 0 I 100
o10 = oo2,1o1,2
e10 = ee2,1e1,2
1 1 II 2
o1,1 = j

e1,1 = a
2 2 II 8
o2,2 = j3

e2,2 = a3
1 2 III 5
o1,2 = p

e1,2 = dp = pd

p
I, II, or III T even ... oa,b ea,b
I or III T odd ... oa,b ea,b oa,b

Examples

Archimedean and Catalan solids

Conway's original set of operators can create all of the

Platonic solids
as seeds. (Note that the r operator is not necessary to create both chiral forms.)

Composite operators

The

face-transitive
.

  • tI
    tI
  • atI
    atI
  • ttI
    ttI
  • ztI = ttD
    ztI = ttD
  • etI
    etI
  • btI
    btI
  • stI
    stI
  • Duals
  • dtI = nI = kD
    dtI = nI = kD
  • jtI
    jtI
  • ntI = kkD
    ntI = kkD
  • ktI
    ktI
  • otI
    otI
  • mtI
    mtI
  • gtI
    gtI

On the plane

Each of the

regular tilings
Q, H, and Δ.

  • Triangle tiling Δ = dH = kH
    Triangle tiling

    Δ = dH = kH
  • Rhombille tiling jΔ = jH
  • Triakis triangular tiling kΔ
    Triakis triangular tiling

  • Deltoidal trihexagonal tiling oΔ = oH
    Deltoidal trihexagonal tiling

    = oH
  • Kisrhombille tiling mΔ = mH
    Kisrhombille tiling

    = mH
  • Floret pentagonal tiling gΔ = gH
    Floret pentagonal tiling

    = gH

On a torus

Conway operators can also be applied to

toroidal polyhedra
and polyhedra with multiple holes.

  • A 1x1 regular square torus, {4,4}1,0
    A 1x1 regular square torus, {4,4}1,0
  • A regular 4x4 square torus, {4,4}4,0
    A regular 4x4 square torus, {4,4}4,0
  • tQ24×12 projected to torus
    tQ24×12 projected to torus
  • taQ24×12 projected to torus
    taQ24×12 projected to torus
  • actQ24×8 projected to torus
    actQ24×8 projected to torus
  • tH24×12 projected to torus
    tH24×12 projected to torus
  • taH24×8 projected to torus
    taH24×8 projected to torus
  • kH24×12 projected to torus
    kH24×12 projected to torus

See also

References

External links

  • polyHédronisme: generates polyhedra in HTML5 canvas, taking Conway notation as input