Tridecagon  In geometry, a tridecagon or triskaidecagon or 13-gon is a thirteen-sided polygon. A "regular tridecagon" is represented by Schläfli symbol {13}. The measure of each internal angle of a regular tridecagon is approximately 152.308 degrees, and the area with side length "a" is given by  As 13 is a Pierpont prime but not a Fermat prime, the regular tridecagon cannot be constructed using a compass and straightedge. However, it is constructible using neusis, or an angle trisector. The following is an animation from a "neusis construction" of a regular tridecagon with radius of circumcircle formula_2 according to Andrew M. Gleason, based on the angle trisection by means of the Tomahawk (light blue). An approximate construction of a regular tridecagon using straightedge and compass is shown here. Another possible animation of an approximate construction, also possible with using straightedge and compass. At a circumscribed circle radius r = 1 billion km (a distance which would take light approximately 55 min to travel), the absolute error of the side length constructed would be < 1 mm. The "regular tridecagon" has Dih13 symmetry, order 26. Since 13 is a prime number there is one subgroup with dihedral symmetry: Dih1, and 2 cyclic group symmetries: Z13, and Z1. These 4 symmetries can be seen in 4 distinct symmetries on the tridecagon. John Conway labels these by a letter and group order. Full symmetry of the regular form is r26 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 g13 subgroup has no degrees of freedom but can seen as directed edges. The regular tridecagon is used as the shape of the Czech 20 korun coin. A tridecagram is a 13-sided star polygon. There are 5 regular forms given by Schläfli symbols: {13/2}, {13/3}, {13/4}, {13/5}, and {13/6}.