Regular Polygon - Regular Star Polygons

Regular Star Polygons

A non-convex regular polygon is a regular star polygon. The most common example is the pentagram, which has the same vertices as a pentagon, but connects alternating vertices.

For an n-sided star polygon, the Schläfli symbol is modified to indicate the density or "starriness" m of the polygon, as {n/m}. If m is 2, for example, then every second point is joined. If m is 3, then every third point is joined. The boundary of the polygon winds around the center m times.

The (non-degenerate) regular stars of up to 12 sides are:

• Pentagram – {5/2}
• Heptagram – {7/2} and {7/3}
• Octagram – {8/3}
• Enneagram – {9/2} and {9/4}
• Decagram – {10/3}
• Hendecagram – {11/2}, {11/3}, {11/4} and {11/5}
• Dodecagram – {12/5}

m and n must be co-prime, or the figure will degenerate.

The degenerate regular stars of up to 12 sides are:

• Hexagram – {6/2}
• Octagram – {8/2}
• Enneagram – {9/3}
• Decagram – {10/2} and {10/4}
• Dodecagram – {12/2}, {12/3} and {12/4}

Depending on the precise derivation of the Schläfli symbol, opinions differ as to the nature of the degenerate figure. For example {6/2} may be treated in either of two ways:

• For much of the 20th century (see for example Coxeter (1948)), we have commonly taken the /2 to indicate joining each vertex of a convex {6} to its near neighbors two steps away, to obtain the regular compound of two triangles, or hexagram.
• Many modern geometers, such as Grünbaum (2003), regard this as incorrect. They take the /2 to indicate moving two places around the {6} at each step, obtaining a "double-wound" triangle that has two vertices superimposed at each corner point and two edges along each line segment. Not only does this fit in better with modern theories of abstract polytopes, but it also more closely copies the way in which Poinsot (1809) created his star polygons – by taking a single length of wire and bending it at successive points through the same angle until the figure closed.