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NORMAL SURFACE

The normal surface associated to a curve traced on a surface is the ruled surface generated by the normals to the surface along the curve.

This surface is developable if and only if the curve is a curvature line of the surface.

Example: the normal surfaces associated to a sphere with
center *O* are the cones with center *O*.

The cuspidal edges of the developable normal surfaces
form the focal surface of the initial
surface.

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© Robert FERRÉOL 2017