# Octadecagon

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{{short description|Polygon with 18 edges}}
{{Regular polygon db|Regular polygon stat table|p18}}
In [geometry](/source/geometry), an '''octadecagon''' (or '''octakaidecagon'''<ref>{{citation|title=Symmetry, Shape, and Surfaces: An Introduction to Mathematics Through Geometry|first1=L. Christine|last1=Kinsey|author1-link=L. Christine Kinsey|first2=Teresa E.|last2=Moore|publisher=Springer|year=2002|isbn=9781930190092|page=86|url=https://books.google.com/books?id=0clfF_CFG9EC&pg=PA86}}.</ref>) or 18-gon is an eighteen-sided [polygon](/source/polygon).<ref>{{citation|title=Cassell's Engineer's Handbook: Comprising Facts and Formulæ, Principles and Practice, in All Branches of Engineering|first=Henry|last=Adams|publisher=D. McKay|year=1907|url=https://books.google.com/books?id=wlopAAAAYAAJ&pg=PA528|page=528}}.</ref>

== Regular octadecagon==
thumb|Octadecagon with all 135 diagonals
A ''[regular](/source/regular_polygon) octadecagon'' has a [Schläfli symbol](/source/Schl%C3%A4fli_symbol) {18} and can be constructed as a quasiregular [truncated](/source/Truncation_(geometry)) [enneagon](/source/enneagon), t{9}, which alternates two types of edges.

===Construction ===
As 18 = 2 × 3<sup>2</sup>, a regular octadecagon cannot be [constructed](/source/constructible_polygon) using a [compass and straightedge](/source/compass_and_straightedge).<ref>{{citation|title=Mathematical Connections: A Capstone Course|first=John B.|last=Conway|author-link=John B. Conway|publisher=American Mathematical Society|year=2010|isbn=9780821849798|page=31|url=https://books.google.com/books?id=iEHVAwAAQBAJ&pg=PA31}}.</ref> However, it is constructible using [neusis](/source/neusis_construction), or an [angle trisection](/source/angle_trisection) with a [tomahawk](/source/Tomahawk_(geometry)).
thumb|400px|left|Octadecagon, an exact construction based on the angle trisection 120° by means of the tomahawk, animation 1 min 34 s.
{{clear}}
The following approximate construction is very similar to that of the enneagon, as an octadecagon can be constructed as a truncated enneagon. It is also feasible with exclusive use of compass and straightedge.

{| class="wikitable"
|-
|style="width:860px" valign="top"|

510px|right
:Downsize the angle AMC (also 60°) with four angle bisectors and make a thirds of circular arc MON with an approximate solution between angle bisectors w<sub>3</sub> and w<sub>4</sub>.
:Straight auxiliary line g aims over the point O to the point N (virtually a ruler at the points O and N applied), between O and N, therefore no auxiliary line.
:Thus, the circular arc MON is freely accessible for the later intersection point R.
::<math>\scriptstyle\angle{}</math> AMR = 19.999999994755615...°
::360° ÷ 18 = 20°
::<math>\scriptstyle\angle{}</math> AMR - 20° = -5.244...E-9°
:''Example to illustrate the error'':
:At a circumscribed circle radius r  = 100,000&nbsp;km, the absolute error of the 1st side would be approximately -9&nbsp;mm.
:See also the [https://de.wikibooks.org/wiki/Mathematik:_Schulmathematik:_Planimetrie:_Polygonkonstruktionen:_Neuneck#Berechnung calculation of nonagon (Berechnung, German)]
:6.0 <math>\scriptstyle\angle{}</math> JMR  equivalent <math>\scriptstyle\angle{}</math> AMR.
|-
|}

== Symmetry==
thumb|320px|Symmetries of a regular octadecagon. Vertices are colored by their symmetry positions. Blue mirrors are drawn through vertices, and purple mirrors are drawn through edge. Gyration orders are given in the center.

The ''regular octadecagon'' has [Dih<sub>18</sub> symmetry](/source/dihedral_symmetry), order 36. There are 5 subgroup dihedral symmetries: Dih<sub>9</sub>, (Dih<sub>6</sub>, Dih<sub>3</sub>), and (Dih<sub>2</sub> Dih<sub>1</sub>), and 6 [cyclic group](/source/cyclic_group) symmetries: (Z<sub>18</sub>, Z<sub>9</sub>), (Z<sub>6</sub>, Z<sub>3</sub>), and (Z<sub>2</sub>, Z<sub>1</sub>).

These 15 symmetries can be seen in 12 distinct symmetries on the octadecagon. [John Conway](/source/John_Horton_Conway) labels these by a letter and group order.<ref>John H. Conway, Heidi Burgiel, [Chaim Goodman-Strauss](/source/Chaim_Goodman-Strauss), (2008) The Symmetries of Things, {{isbn|978-1-56881-220-5}} (Chapter 20, Generalized Schaefli symbols, Types of symmetry of a polygon pp. 275-278)</ref> Full symmetry of the regular form is '''r36''' and no symmetry is labeled '''a1'''. The dihedral symmetries are divided depending on whether they pass through vertices ('''d''' for diagonal) or edges ('''p''' for perpendiculars), and '''i''' when reflection lines path through both edges and vertices. Cyclic symmetries in the middle column are labeled as '''g''' for their central gyration orders.

Each subgroup symmetry allows one or more degrees of freedom for irregular forms. Only the '''g18''' subgroup has no degrees of freedom but can be seen as [directed edge](/source/directed_edge)s.

{{-}}
== Dissection==
thumb|18-gon with 144 rhombs
[[File:Equilateral_pentagonal_dissection_of_regular_octadecagon.svg|thumb|An [equilateral pentagon](/source/equilateral_pentagon)al [dissection](/source/Pentagonal_tiling), with sequential internal angles: 60°, 160°, 80°, 100°, and 140°. Each of the 24 pentagons can be seen as the union of an [equilateral triangle](/source/equilateral_triangle) and an 80° [rhombus](/source/rhombus).{{sfn|Hirschhorn|Hunt|1985}}]]
[Coxeter](/source/Coxeter) states that every [zonogon](/source/zonogon) (a 2''m''-gon whose opposite sides are parallel and of equal length) can be dissected into ''m''(''m'' − 1)/2 parallelograms.<ref>[Coxeter](/source/Coxeter), Mathematical recreations and Essays, Thirteenth edition, p. 141</ref>
In particular this is true for regular polygons with evenly many sides, in which case the parallelograms are all rhombi. For the ''regular octadecagon'', ''m'' = 9, and it can be divided into 36: 4 sets of 9 rhombs. This decomposition is based on a [Petrie polygon](/source/Petrie_polygon) projection of a [9-cube](/source/9-cube), with 36 of 4608 faces. The list {{OEIS2C|1=A006245}} enumerates the number of solutions as 112018190, including up to 18-fold rotations and chiral forms in reflection.
{| class=wikitable
|+ Dissection into 36 rhombs
|- align=center valign=top
|160px|class=skin-invert
|160px
|160px
|160px
|160px
|}

==Uses==
160px<BR>A regular triangle, nonagon, and octadecagon can completely surround a point in the plane, one of 17 different combinations of regular polygons with this property.<ref>{{citation|title=The Elements of Plane Practical Geometry, Etc|first=Elmslie William|last=Dallas|publisher=John W. Parker & Son|year=1855|page=134|url=https://books.google.com/books?id=y4BaAAAAcAAJ&pg=PA134}}.</ref> However, this pattern cannot be extended to an [Archimedean tiling](/source/Tiling_by_regular_polygons) of the plane: because the triangle and the nonagon both have an odd number of sides, neither of them can be completely surrounded by a ring alternating the other two kinds of polygon.

The regular octadecagon can tessellate the plane with concave hexagonal gaps. And another tiling mixes in nonagons and octagonal gaps. The first tiling is related to a [truncated hexagonal tiling](/source/truncated_hexagonal_tiling), and the second the [truncated trihexagonal tiling](/source/truncated_trihexagonal_tiling).
:240px 240px

==Related figures==
An '''octadecagram''' is an 18-sided star polygon, represented by symbol {18/n}. There are two regular [star polygon](/source/star_polygon)s: {18/5} and {18/7}, using the same points, but connecting every fifth or seventh points. There are also five compounds: {18/2} is reduced to 2{9} or two [enneagon](/source/enneagon)s, {18/3} is reduced to 3{6} or three [hexagon](/source/hexagon)s, {18/4} and {18/8} are reduced to 2{9/2} and 2{9/4} or two [enneagram](/source/enneagram_(geometry))s, {18/6} is reduced to 6{3} or 6 equilateral triangles, and finally {18/9} is reduced to 9{2} as nine [digon](/source/digon)s.

{| class="wikitable collapsible collapsed"
!colspan=10|Compounds and star polygons
|-
!n||1||2||3||4||5||6||7||8||9
|-
!Form
!Convex polygon
!colspan=3|Compounds
!Star polygon
!Compound
!Star polygon
!colspan=2|Compound
|- align=center valign=top
!valign=center|Image
|BGCOLOR="#ffe0e0"|80px<BR>{18/1}<BR>= {18}
|80px<br>{18/2}<BR>= 2{9}
|80px<br>{18/3}<BR>= 3{6}
|80px<br>{18/4}<BR>= 2{9/2}
|BGCOLOR="#ffe0e0"|80px<br>{18/5}
|80px<br>{18/6}<BR>= 6{3}
|BGCOLOR="#ffe0e0"|80px<br>{18/7}
|80px<br>{18/8}<BR>= 2{9/4}
|80px<br>{18/9}<BR>= 9{2}
|- align=center
! [Interior angle](/source/Interior_angle)
| 160°
| 140°
| 120°
| 100°
| 80°
| 60°
| 40°
| 20°
| 0°
|}

Deeper truncations of the regular enneagon and enneagrams can produce isogonal ([vertex-transitive](/source/vertex-transitive)) intermediate octadecagram forms with equally spaced vertices and two edge lengths. Other truncations form double coverings: t{9/8}={18/8}=2{9/4}, t{9/4}={18/4}=2{9/2}, t{9/2}={18/2}=2{9}.<ref>The Lighter Side of Mathematics: Proceedings of the Eugène Strens Memorial Conference on Recreational Mathematics and its History, (1994), ''Metamorphoses of polygons'', [Branko Grünbaum](/source/Branko_Gr%C3%BCnbaum)</ref>
{| class="wikitable collapsible collapsed"
!colspan=6|Vertex-transitive truncations of enneagon and enneagrams
|-
!Quasiregular
!colspan=4|isogonal
!Quasiregular<BR>Double covering
|- align=center valign=top
|BGCOLOR="#ffe0e0"|80px<BR>t{9}={18}
|80px
|80px
|80px
|80px
|BGCOLOR="#e0e0ff"|80px<BR>t{9/8}={18/8}<BR>=2{9/4}
|- align=center valign=top
|BGCOLOR="#ffe0e0"|80px<BR>t{9/5}={18/5}
|80px
|80px
|80px
|80px
|BGCOLOR="#e0e0ff"|80px<BR>t{9/4}={18/4}<BR>=2{9/2}
|- align=center valign=top
|BGCOLOR="#ffe0e0"|80px<BR>t{9/7}={18/7}
|80px
|80px
|80px
|80px
|BGCOLOR="#e0e0ff"|80px<BR>t{9/2}={18/2}<BR>=2{9}
|}

===Petrie polygons===
A regular skew octadecagon is the [Petrie polygon](/source/Petrie_polygon) for a number of higher-dimensional polytopes, shown in these skew [orthogonal projection](/source/orthogonal_projection)s from [Coxeter plane](/source/Coxeter_plane)s:

{| class="wikitable collapsible collapsed"
!colspan=8|Octadecagonal petrie polygons
|-
!A<sub>17</sub>
!colspan=2|B<sub>9</sub>
!colspan=2|D<sub>10</sub>
!colspan=3|E<sub>7</sub>
|- align=center
|80px<br>[17-simplex](/source/17-simplex)
|80px<br>[9-orthoplex](/source/9-orthoplex)
|80px<br>[9-cube](/source/9-cube)
|80px<br>[7<sub>11</sub>](/source/10-orthoplex)
|80px<br>[1<sub>71</sub>](/source/10-demicube)
|80px<br>[3<sub>21</sub>](/source/E7_polytope)
|80px<br>[2<sub>31</sub>](/source/Gosset_2_31_polytope)
|80px<br>[1<sub>32</sub>](/source/Gosset_1_32_polytope)
|}

==References==
{{reflist}}
* {{Citation | last1=Hirschhorn | first1=M. D. | last2=Hunt | first2=D. C. | title=Equilateral convex pentagons which tile the plane | doi=10.1016/0097-3165(85)90078-0 | mr=787713 | year=1985 | journal=Journal of Combinatorial Theory, Series A | issn=1096-0899 | volume=39 | issue=1 | pages=1–18| doi-access=free |url=https://core.ac.uk/download/pdf/82754854.pdf |access-date=2020-10-30}}
*[https://books.google.com/books?id=ugBDAAAAIAAJ&dq=octadecagon&pg=PA194 octadecagon]

==External links==
*{{MathWorld|title=Octadecagon|urlname=Octadecagon}}

{{Polygons}}

Category:Polygons by the number of sides

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Adapted from the Wikipedia article [Octadecagon](https://en.wikipedia.org/wiki/Octadecagon) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Octadecagon?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.