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The Info List - Petrie Polygon



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In geometry , a PETRIE POLYGON for a regular polytope of n dimensions is a skew polygon such that every (n-1) consecutive side (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 .

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

CONTENTS

* 1 History * 2 The Petrie polygons of the regular polyhedra * 3 The Petrie polygon
Petrie polygon
of regular polychora (4-polytopes) * 4 The Petrie polygon
Petrie polygon
projections of regular and uniform polytopes * 5 Notes * 6 References * 7 External links

HISTORY

The Petrie polygon
Petrie polygon
for a cube is a skew hexagon passing through 6 of 8 vertices. The skew Petrie polygon
Petrie polygon
can be seen as a regular planar polygon by a specific orthogonal projection .

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
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
The Fifty-Nine Icosahedra
for publication. Realizing the geometric facility of the skew polygons used by Petrie, Coxeter
Coxeter
named them after his friend when he wrote Regular Polytopes .

In 1972, a few months after his retirement, Petrie was killed by a car while attempting to cross a motorway near his home in Surrey
Surrey
.

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

THE PETRIE POLYGONS OF THE REGULAR POLYHEDRA

The Petrie polygon
Petrie polygon
of the regular polyhedron {p, q} has h sides, where h+2=24/(10-p-q).

The regular duals , {p,q} and {q,p}, are contained within the same projected Petrie polygon.

Petrie polygons for regular polyhedra (red polygons) SQUARE HEXAGON DECAGON

tetrahedron cube octahedron dodecahedron icosahedron

edge-centered vertex-centered face-centered face-centered vertex-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. Blue show "front" edges, while black lines show back edges.

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.

Three of the Kepler–Poinsot polyhedra have hexagonal , {6}, and decagrammic , {10/3}, petrie polygons.

Kepler–Poinsot polyhedra HEXAGON DECAGRAM

{5,5/2} {5,5/2} {5/2,3} {3,5/2}

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

{3,3,3}

5-cell 5 sides V:(5,0) {3,3,4}

16-cell 8 sides V:(8,0) {4,3,3}

tesseract 8 sides V:(8,8,0)

{3,4,3}

24-cell 12 sides V:(12,6,6,0) {5,3,3}

120-cell 30 sides V:((30,60)3,603,30,60,0) {3,3,5}

600-cell 30 sides V:(30,30,30,30,0)

THE PETRIE POLYGON PROJECTIONS OF REGULAR AND UNIFORM POLYTOPES

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

I2(p) Dn

E6 E7 E8 F4 G2

HN

2

Triangle

Square
Square

p-gon (example: p=7 )

Hexagon
Hexagon

Pentagon
Pentagon

3

Tetrahedron
Tetrahedron

Cube
Cube

Octahedron
Octahedron

Tetrahedron
Tetrahedron

Dodecahedron
Dodecahedron

Icosahedron
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-cube

8-orthoplex
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

NOTES

* ^ KALEIDOSCOPES: SELECTED WRITINGS OF H. S. M. COXETER, edited by F. Arthur Sherk, Peter McMullen , Anthony C. Thompson, Asia Ivic Weiss, Wiley-Interscience Publication, 1995, ISBN 978-0-471-01003-6 (Definition: paper 13, Discrete groups generated by reflections, 1933, p. 161) * ^ H.S.M. Coxeter
Coxeter
(1937) "Regular skew polyhedral in three and four dimensions and their topological analogues", Proceedings of the London Mathematical Society (2) 43: 33 to 62 * ^ H. S. M. Coxeter, Patrick du Val , H.T. Flather, J.F. Petrie (1938) The Fifty-nine Icosahedra, University of Toronto
University of Toronto
studies, mathematical series 6: 1–26 * ^ H.S.M. Coxeter
Coxeter
(1973) Regular Polytopes , 3rd edition, page 32 * ^ http://cms.math.ca/openaccess/cjm/v10/cjm1958v10.0220-0221.pdf

REFERENCES

* Coxeter
Coxeter
, H. S. M. (1947, 63, 73) Regular Polytopes , 3rd ed. New York: Dover, 1973. (sec 2.6 Petrie Polygons pp. 24–25, and Chapter 12, pp. 213–235, The generalized Petrie polygon
Petrie polygon
) * Coxeter, H.S.M. (1974) Regular complex polytopes. Section 4.3 Flags and Orthoschemes, Section 11.3 Petrie polygons * Ball, W. W. R. and H. S. M. Coxeter
Coxeter
(1987) Mathematical Recreations and Essays, 13th ed. New York: Dover. (p. 135) * Coxeter, H. S. M. (1999) The Beauty of Geometry: Twelve Essays, Dover Publications LCCN 99-35678 * Peter McMullen , Egon Schulte (2002) Abstract Regular Polytopes, Cambridge University Press
Cambridge University Press
. ISBN 0-521-81496-0 * Steinberg, Robert,ON THE NUMBER OF SIDES OF A PETRIE POLYGON

EXTERNAL

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