# Petrie polygon

In geometry, a Petrie polygon for a regular polytope of n dimensions is a skew polygon in which every (n  1) consecutive sides (but no n) belongs to one of the facets. The Petrie polygon of a regular polygon is the regular polygon itself; that of a regular polyhedron is a skew polygon such that every two consecutive side (but no three) belongs to one of the faces.

The Petrie polygon of the dodecahedron is a skew decagon. Seen from the solid's 5-fold symmetry axis it looks like a regular decagon. Every pair of consecutive sides belongs to one pentagon (but no triple does).

For every regular polytope there exists an orthogonal projection onto a plane such that one Petrie polygon becomes a regular polygon with the remainder of the projection interior to it. The plane in question is the Coxeter plane of the symmetry group of the polygon, and the number of sides, h, is Coxeter number of the Coxeter group. These polygons and projected graphs are useful in visualizing symmetric structure of the higher-dimensional regular polytopes.

Petrie polygons can be defined more generally for any embedded graph. They form the faces of another embedding of the same graph, usually on a different surface, called the Petrie dual.

## History

John Flinders Petrie (1907–1972) was the only son of Egyptologist Flinders Petrie. He was born in 1907 and as a schoolboy showed remarkable promise of mathematical ability. In periods of intense concentration he could answer questions about complicated four-dimensional objects by visualizing them.

He first noted the importance of the regular skew polygons which appear on the surface of regular polyhedra and higher polytopes. Coxeter explained in 1937 how he and Petrie began to expand the classical subject of regular polyhedra:

One day in 1926, J. F. Petrie told me with much excitement that he had discovered two new regular polyhedral; infinite but free of false vertices. When my incredulity had begun to subside, he described them to me: one consisting of squares, six at each vertex, and one consisting of hexagons, four at each vertex.

In 1938 Petrie collaborated with Coxeter, Patrick du Val, and H.T. Flather to produce The Fifty-Nine Icosahedra for publication. Realizing the geometric facility of the skew polygons used by Petrie, Coxeter named them after his friend when he wrote Regular Polytopes.

The idea of Petrie polygons was later extended to semiregular polytopes.

## The Petrie polygons of the regular polyhedra

Two tetrahedra with Petrie squares
Cube and octahedron with Petrie hexagons
Dodecahedron and icosahedron with Petrie decagons

The regular duals, {p,q} and {q,p}, are contained within the same projected Petrie polygon. In the images of dual compounds on the right it can be seen that their Petrie polygons have rectangular intersections in the points where the edges touch the common midsphere.

Petrie polygons for Platonic solids
SquareHexagonDecagon
tetrahedron {3,3} cube {4,3} octahedron {3,4} dodecahedron {5,3} icosahedron {3,5}
edge-centeredvertex-centeredface-centeredface-centeredvertex-centered
V:(4,0)V:(6,2)V:(6,0)V:(10,10,0)V:(10,2)

The Petrie polygons are the exterior of these orthogonal projections.
The concentric rings of vertices are counted starting from the outside working inwards with a notation: V:(a, b, ...), ending in zero if there are no central vertices.
The number of sides for {p, q} is 24/(10−pq) − 2.

gD and sD with Petrie hexagons
gI and gsD with Petrie decagrams

The Petrie polygons of the Kepler–Poinsot polyhedra are hexagons {6} and decagrams {10/3}.

Petrie polygons for Kepler–Poinsot polyhedra
HexagonDecagram
gD {5,5/2} sD {5,5/2} gI {3,5/2} gsD {5/2,3}

Infinite regular skew polygons (apeirogon) can also be defined as being the Petrie polygons of the regular tilings, having angles of 90, 120, and 60 degrees of their square, hexagon and triangular faces respectively.

Infinite regular skew polygons also exist as Petrie polygons of the regular hyperbolic tilings, like the order-7 triangular tiling, {3,7}:

## The Petrie polygon of regular polychora (4-polytopes)

The Petrie polygon of the tesseract is an octagon. Every triple of consecutive sides belongs to one of its eight cubic cells.

The Petrie polygon for the regular polychora {p, q ,r} can also be determined.

 {3,3,3}5-cell5 sidesV:(5,0) {3,3,4}16-cell8 sidesV:(8,0) {4,3,3}tesseract8 sidesV:(8,8,0) {3,4,3}24-cell12 sidesV:(12,6,6,0) {5,3,3}120-cell30 sidesV:((30,60)3,603,30,60,0) {3,3,5}600-cell30 sidesV:(30,30,30,30,0)

## The Petrie polygon projections of regular and uniform polytopes

The Petrie polygon projections are useful for the visualization of polytopes of dimension four and higher.

### Hypercubes

A hypercube of dimension n has a Petrie polygon of size 2n, which is also the number of its facets.
So each of the (n−1)-cubes forming its surface has n−1 sides of the Petrie polygon among its edges.

### Irreducible polytope families

This table represents Petrie polygon projections of 3 regular families (simplex, hypercube, orthoplex), and the exceptional Lie group En which generate semiregular and uniform polytopes for dimensions 4 to 8.

Table of irreducible polytope families
Family
n
n-simplex n-hypercube n-orthoplex n-demicube 1k2 2k1 k21 pentagonal polytope
Group An Bn
 I2(p) Dn
 E6 E7 E8 F4 G2
Hn
2

Triangle

Square

p-gon
(example: p=7)

Hexagon

Pentagon
3

Tetrahedron

Cube

Octahedron

Tetrahedron

Dodecahedron

Icosahedron
4

5-cell

Tesseract

16-cell

Demitesseract

24-cell

120-cell

600-cell
5

5-simplex

5-cube

5-orthoplex

5-demicube

6

6-simplex

6-cube

6-orthoplex

6-demicube

122

221

7

7-simplex

7-cube

7-orthoplex

7-demicube

132

231

321

8

8-simplex

8-cube

8-orthoplex

8-demicube

142

241

421

9

9-simplex

9-cube

9-orthoplex

9-demicube

10

10-simplex

10-cube

10-orthoplex

10-demicube

Various visualizations of the icosahedron
Fundamental convex regular and uniform polytopes in dimensions 2–10
Family An Bn I2(p) / Dn E6 / E7 / E8 / F4 / G2 Hn
Regular polygon Triangle Square p-gon Hexagon Pentagon
Uniform polyhedron Tetrahedron OctahedronCube Demicube DodecahedronIcosahedron
Uniform 4-polytope 5-cell 16-cellTesseract Demitesseract 24-cell 120-cell600-cell
Uniform 5-polytope 5-simplex 5-orthoplex5-cube 5-demicube
Uniform 6-polytope 6-simplex 6-orthoplex6-cube 6-demicube 122221
Uniform 7-polytope 7-simplex 7-orthoplex7-cube 7-demicube 132231321
Uniform 8-polytope 8-simplex 8-orthoplex8-cube 8-demicube 142241421
Uniform 9-polytope 9-simplex 9-orthoplex9-cube 9-demicube
Uniform 10-polytope 10-simplex 10-orthoplex10-cube 10-demicube
Uniform n-polytope n-simplex n-orthoplexn-cube n-demicube 1k22k1k21 n-pentagonal polytope
Topics: Polytope familiesRegular polytopeList of regular polytopes and compounds