Whitney's Umbrella may be pictured as a self-intersecting rectangle in 3 dimensions. It demonstrates a pinch point, which occurs at the top endpoint of the segment of self-intersection. Every neighborhood of the pinch point intersects itself. Pinch points are also called Whitney singularities or branch points.
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Created: Jul 6 1995 ---- Last modified: Tue Oct 10 21:51:19 1995
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