- #76

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Then how to explain the fact that the line element ##ds^2 = (1+ y^2/(R^2 - y^2))dy^2## does correspond to ##ds^2 = R^2 d\theta^2## for the surface of a sphere, with ##z = 0##? Or maybe I'm wrong about the definition of a metric?If the metric does not have an inverse it is not a metric.