For a minimal surface, the shape operator has eigenvalues , so . Conversely this metric identity only implies that the two principal curvatures have equal squares. A sphere has equal nonzero principal curvatures and satisfies the identity while having nonzero mean curvature. The distinction is between opposite and equal signs, not equality of magnitudes.
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The Gauss map is a mathematical construct used primarily in differential geometry. It associates a surface in three-dimensional space with a unit normal vector at each point of the surface. More specifically, the Gauss map sends each point on a surface to the corresponding point on the unit sphere that represents the normal vector at that point.