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Pinch point (mathematics) : ウィキペディア英語版
Pinch point (mathematics)

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
: f(u,v,w) = u^2 - vw^2 + () \,
where () denotes terms of degree 4 or more and v is not a square in the ring of functions.
For example the surface 1-2x+x^2-yz^2=0 near the point (1,0,0), meaning in coordinates vanishing at that point, has the form above. In fact, if u=1-x, v=y and w=z then is a system of coordinates vanishing at (1,0,0) then 1-2x+x^2-yz^2=(1-x)^2-yz^2=u^2-vw^2 is written in the canonical form.
The simplest example of a pinch point is the hypersurface defined by the equation u^2-vw^2=0 called Whitney umbrella.
The pinch point (in this case the origin) is a limit of normal crossings singular points (the v-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 v-axis and not only the pinch point.
==See also==

*Whitney umbrella
*Singular point of an algebraic variety

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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