![](http://upload.wikimedia.org/wikipedia/commons/7/79/Whitney_unbrella.png)
In geometry, a pinch point or cuspidal point is a type of singular point on an algebraic surface.
The equation for the surface near a pinch point may be put in the form
where [4] denotes terms of degree 4 or more and is not a square in the ring of functions.
For example the surface near the point , meaning in coordinates vanishing at that point, has the form above. In fact, if and then {} is a system of coordinates vanishing at then is written in the canonical form.
The simplest example of a pinch point is the hypersurface defined by the equation called Whitney umbrella.
The pinch point (in this case the origin) is a limit of normal crossings singular points (the -axis in this case). These singular points are intimately related in the sense that in order to resolve the pinch point singularity one must blow-up the whole -axis and not only the pinch point.
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See also
References
- P. Griffiths; J. Harris (1994). Principles of Algebraic Geometry. Wiley Classics Library. Wiley Interscience. pp. 23–25. ISBN 0-471-05059-8.
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