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quadratic set : ウィキペディア英語版
quadratic set

In mathematics, a quadratic set is a set of points in a projective plane/space which bears the same essential incidence properties as a quadric (conic section in a projective plane, sphere or cone or hyperboloid in a projective space).
== Definition of a quadratic set==
Let be \mathfrak P=(,,\in) a projective space. A non empty subset of is called quadratic set if
:(QS1) Any line g of intersects in at most 2 points or is contained in .
:(g is called exterior, tangent and secant line if |g\cap |=0, \ |g\cap |=1 and |g\cap |=2 respectively.)
:(QS2) For any point P\in the union _P of all tangent lines through P is a hyperplane or the entire space .
A quadratic set is called non degenerated if for any point P set
_P is a hyperplane.
The following result is an astonishing statement for finite projective spaces.
Theorem(BUEKENHOUT):
Let be \mathfrak P_n a ''finite'' projective space of dimension n\ge 3 and
a non degenerated quadratic set which contains lines. Then:
\mathfrak P_n is pappian and is a ''quadric'' with index \ge 2.

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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