A regular hendecagon
|Edges and vertices||11|
|Symmetry group||Dihedral (D11), order 2×11|
|Internal angle (degrees)||≈147.273°|
|Properties||convex, cyclic, equilateral, isogonal, isotoxal|
In geometry, a hendecagon (also undecagon or endecagon) is an eleven-sided polygon or 11-gon. (The name hendecagon, from Greek hendeka "eleven" and gon– "corner", is often preferred to the hybrid undecagon, whose first part is formed from Latin undecim "eleven".)
As 11 is not a Fermat prime, the regular hendecagon is not constructible with compass and straightedge. Because 11 is not a Pierpont prime, construction of a regular hendecagon is still impossible even with the usage of an angle trisector.
Close approximations to the regular hendecagon can be constructed, however. For instance, the ancient Greek mathematicians approximated the side length of a hendecagon inscribed in a unit circle as being 14/25 units long.
These 4 symmetries can be seen in 4 distinct symmetries on the hendecagon. John Conway labels these by a letter and group order. Full symmetry of the regular form is r22 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 g11 subgroup has no degrees of freedom but can seen as directed edges.
Use in coinage
The Canadian dollar coin, the loonie, is similar to, but not exactly, a regular hendecagonal prism, as are the Indian 2-rupee coin and several other lesser-used coins of other nations. The cross-section of a loonie is actually a Reuleaux hendecagon. The United States Susan B. Anthony dollar has a hendecagonal outline along the inside of its edges.
The hendecagon shares the same set of 11 vertices with four regular hendecagrams:
- 10-simplex - can be seen as a complete graph in a regular hendecagonal orthogonal projection
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- Hendecagon – from Wolfram MathWorld
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- As Gauss proved, a polygon with a prime number p of sides can be constructed if and only if p − 1 is a power of two, not true for 11. See Kline, Morris (1990), Mathematical Thought From Ancient to Modern Times 2, Oxford University Press, pp. 753–754, ISBN 9780199840427.
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- U.S. House of Representatives, 1978, p. 7.
- Properties of an Undecagon (hendecagon) With interactive animation
- Weisstein, Eric W., "Hendecagon", MathWorld.