In mathematics, in geometry, a **cyclogon** is the curve traced by a vertex of a polygon that rolls without slipping along a straight line.^{[1]}^{[2]} There are no restrictions on the nature of the polygon. It can be a regular polygon like an equilateral triangle or a square. The polygon need not even be convex: it could even be a star-shaped polygon. More generally, the curves traced by points other than vertices have also been considered. In such cases it would be assumed that the tracing point is rigidly attached to the polygon. If the tracing point is located outside the polygon, then the curve is called a *prolate cyclogon*, and if it lies inside the polygon it is called a *curtate cyclogon*.

In the limit, as the number of sides increases to infinity, the cyclogon becomes a cycloid.^{[3]}

The cyclogon has an interesting property regarding its area.^{[3]} Let *A* denote the area of the region above the line and below one of the arches, let *P* denote the area of the rolling polygon, and let *C* denote the area of the disk that circumscribes the
polygon. For every cyclogon generated by a regular polygon,

## Examples

### Cyclogons generated by an equilateral triangle and a square

### Prolate cyclogon generated by an equilateral triangle

### Curtate cyclogon generated by an equilateral triangle

### Cyclogons generated by quadrilaterals

## Generalized cyclogons

A cyclogon is obtained when a polygon rolls over a straight line. Let it be assumed that the regular polygon rolls over the edge of another polygon. Let it also be assumed that the tracing point is not a point on the boundary of the polygon but possibly a point within the polygon or outside the polygon but lying in the plane of the polygon. In this more general situation, let a curve be traced by a point z on a regular polygonal disk with n sides rolling around another regular polygonal disk with m sides. The edges of the two regular polygons are assumed to have the same length. A point z attached rigidly to the n-gon traces out an arch consisting of n circular arcs before repeating the pattern periodically. This curve is called a **trochogon** — an *epitrochogon* if the n-gon rolls outside the m-gon, and a *hypotrochogon* if it rolls inside the m-gon. The trochogon is curtate if z is inside the n-gon, and prolate (with loops) if z is outside the n-gon. If z is at a vertex it traces an epicyclogon or a hypocyclogon.^{[4]}

## See also

## References

**^**Tom M. Apostol, Mamikon Mnatsakanian (2012).*New Horizons in Geometry*. Mathematical Association of America. p. 68. ISBN 9780883853542.**^**Ken Caviness. "Cyclogons".*Wolfram Demonstrations Project*. Retrieved 23 December 2015.- ^
^{a}^{b}T. M. Apostol and M. A. Mnatsakanian (1999). "Cycloidal Areas without Calculus" (PDF).*Math Horizons*.**7**(1): 12–16. Archived from the original (PDF) on 2005-01-30. Retrieved 23 December 2015. **^**Tom M. Apostopl and Mamikon A. Mnatsaknian (September 2002). "Generalized Cyclogons" (PDF).*Math Horizons*. Archived from the original (PDF) on 2005-01-30. Retrieved 23 December 2015.