Delta Curve
A curve which can be turned continuously inside an Equilateral Triangle. There are an infinite number of deltacurves, but the simplest are the Circle and lens-shaped 
-biangle. All the curves of height 
havethe same Perimeter 
. Also, at each position of a curve turning in an Equilateral Triangle,the perpendiculars to the sides at the points of contact are Concurrent at the instantaneous center of rotation.
References
Honsberger, R. Mathematical Gems I. Washington, DC: Math. Assoc. Amer., pp. 56-59, 1973.
- Principal
- Principal Curvatures
- Principal Curve
- Principal Direction
- Principal Ideal
- Principal Normal Vector
- Principal Quintic Form
- Principal Radius of Curvature
- Principal Value
- Principal Vector
- Principal Vertex
- Principle
- Pringsheim's Theorem
- Printer's Errors
- Prior Distribution
- Prism
- Prismatic Ring
- Prismatoid
- Prismoid
- Prisoner's Dilemma
- Prizes
- Probability
- Probability Axioms
- Probability Density Function
- Probability Distribution Function