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Intersection theorem : ウィキペディア英語版
Intersection theorem
In projective geometry, an intersection theorem or incidence theorem is a statement concerning an incidence structure – consisting of points, lines, and possibly higher-dimensional objects and their incidences – together with a pair of objects and (for instance, a point and a line). The "theorem" states that, whenever a set of objects satisfies the incidences (''i.e.'' can be identified with the objects of the incidence structure in such a way that incidence is preserved), then the objects and must also be incident. An intersection theorem is not necessarily true in all projective geometries; it is a property that some geometries satisfy but others don't.
For example, Desargues' theorem can be stated using the following incidence structure:
*Points: \
*Lines: \
*Incidences (in addition to obvious ones such as (A,AB)): \
The implication is then (R,PQ)—that point is incident with line .
== Famous examples ==
Desargues' theorem holds in a projective plane if and only if is the projective plane over some division ring (skewfield} — P=\mathbb_D. The projective plane is then called ''desarguesian''.
A theorem of Amitsur and Bergman states that, in the context of desarguesian projective planes, for every intersection theorem there is a rational identity such that the plane satisfies the intersection theorem if and only if the division ring satisfies the rational identity.
*Pappus's hexagon theorem holds in a desarguesian projective plane \mathbb_D if and only if is a field; it corresponds to the identity \forall a,b\in D, \quad a\cdot b=b\cdot a.
*Fano's axiom (which states a certain intersection does ''not'' happen) holds in \mathbb_D if and only if has characteristic \neq 2; it corresponds to the identity .

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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