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a epicycloyd. a roulette traced by a point attached to a [circle of radius r rolling around the outside of a fixed circle of radius R, where the point is at a distance d from the center of the exterior circle (from Google);or, in short: a roulette rolling with a R radius in a fixed outer circle, with the roulette itself being a "r" one, rolling freely inside a fixed last one. Like in a Mazdarotary engine
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