Decagram (geometry)
| Regular decagram | |
|---|---|
A regular decagram
|
|
| Type | Regular polygon |
| Edges and vertices | 10 |
| Schläfli symbol | {10/3} t{5/3} |
| Coxeter diagram |       {{{p10/3-CD2}}} |
| Symmetry group | Dihedral (D10) |
| Internal angle (degrees) | 72° |
| Dual polygon | self |
| Properties | star, cyclic, equilateral, isogonal, isotoxal |
In geometry, a decagram is a 10-point star polygon. There is one regular decagram, containing the vertices of a regular decagon, but connected by every third point. Its Schläfli symbol is {10/3}.[1]
The name decagram combine a numeral prefix, deca-, with the Greek suffix -gram. The -gram suffix derives from γραμμῆς (grammēs) meaning a line.[2]
Regular decagram
For a regular decagram with unit edge lengths, the proportions of the crossing points on each edge are as shown below.
Applications
Decagrams have been used as one of the decorative motifs in girih tiles.[3]
Related figures
A regular decagram is a 10-sided polygram, represented by symbol {10/n}, containing the same vertices as regular decagon. Only one of these polygrams, {10/3} (connecting every third point), forms a regular star polygon, but there are also three ten-vertex polygrams which can be interpreted as regular compounds:
- {10/5} is a compound of five degenerate digons 5{2}
- {10/4} is a compound of two pentagrams 2{5/2}
- {10/2} is a compound of two pentagons 2{5}.[4][5]
| Form | Convex | Compound | Star polygon | Compounds | |
|---|---|---|---|---|---|
| Image |  |
.svg) |
|
.svg) |
.svg) |
| Symbol | {10/1} = {10} | {10/2} = 2{5} | {10/3} | {10/4} = 2{5/2} | {10/5} = 5{2} |
Deeper truncations of the regular pentagon and pentagram can produce intermediate star polygon forms with ten equally spaced vertices and two edge lengths that remain vertex-transitive (any two vertices can be transformed into each other by a symmetry of the figure).[6][7][8]
| Quasiregular | Isogonal | Quasiregular Double covering |
|
|---|---|---|---|
 t{5} = {10} |
 |
 |
 t{5/4} = {10/4} = 2{5/2} |
 t{5/3} = {10/3} |
 |
 |
 t{5/2} = {10/2} = 2{5} |
See also
References
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Wikimedia Commons has media related to Ten pointed stars. |
- ↑ Lua error in package.lua at line 80: module 'strict' not found..
- ↑ γραμμή, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
- ↑ Lua error in package.lua at line 80: module 'strict' not found..
- ↑ Regular polytopes, p 93-95, regular star polygons, regular star compounds
- ↑ Coxeter, Introduction to Geometry, second edition, 2.8 Star polygons p.36-38
- ↑ The Lighter Side of Mathematics: Proceedings of the Eugène Strens Memorial Conference on Recreational Mathematics and its History, (1994), Metamorphoses of polygons, Branko Grünbaum.
- ↑ *Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Coxeter, The Densities of the Regular polytopes I, p.43 If d is odd, the truncation of the polygon {p/q} is naturally {2n/d}. But if not, it consists of two coincident {n/(d/2)}'s; two, because each side arises from an original side and once from an original vertex. Thus the density of a polygon is unaltered by truncation.
