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| | astroid |
 | | The astroid was first studied by the Danish astronomer Ole Römer in 1674, in his search for better shapes for gear teeth, and later by Johann Bernoulli (1691), Gottfried Leibniz (1715), and Jean d'Alembert (1748). |
 | | It has a remarkable relationship with the quadrifolium: the radial, pedal, and orthoptic of the astroid are the quadrifolium, while the catacaustic (see caustic curve) of the quadrifolium is the astroid. |
 | | The astroid is also the catacaustic of the deltoid and the evolute of the ellipse. |
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