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A convex figure that can be rotated inside a polygon (or polyhedron) while always touching every side (or face). The least area rotor in a square is the Reuleaux triangle. The least area rotor in an equilateral triangle is a lens with two 60° arcs of circles and radius equal to the triangle altitude. There exist nonspherical rotors for the tetrahedron, octahedron, and cube, but not for the dodecahedron and icosahedron.

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   • curve of constant width

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