A coordinate system similar to toroidal coordinates but with fourth-degree instead of second-degree surfaces for constant so that the toroids of circular cross section are replaced by flattened rings, and the spherical bowls are replaced by cyclides of rotation for constant . The transformation equations are
(1)
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(2)
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(3)
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where
(4)
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and with , , and . Surfaces of constant are given by the flat-ring cyclides
(5)
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surfaces of constant by the cyclides of rotation
(6)
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and surfaces of constant by the half-planes
(7)
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