# 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 A and B (per esempio, a point and a line). Il "teorema" afferma che, whenever a set of objects satisfies the incidences (cioè. can be identified with the objects of the incidence structure in such a way that incidence is preserved), then the objects A and B 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.

Per esempio, Desargues' theorem can be stated using the following incidence structure: Points: {stile di visualizzazione {UN,B,C,un,b,c,P,Q,R,o}} Lines: {stile di visualizzazione {AB,corrente alternata,AVANTI CRISTO,ab,ac,avanti Cristo,Aa,Bb,Cc,PQ}} Incidences (in addition to obvious ones such as {stile di visualizzazione (UN,AB)} ): {stile di visualizzazione {(o,Aa),(o,Bb),(o,Cc),(P,AVANTI CRISTO),(P,avanti Cristo),(Q,corrente alternata),(Q,ac),(R,AB),(R,ab)}} The implication is then {stile di visualizzazione (R,PQ)} —that point R is incident with line PQ.

Famous examples Desargues' theorem holds in a projective plane P if and only if P is the projective plane over some division ring (skewfield} D — {displaystyle P=mathbb {P} _{2}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 P satisfies the intersection theorem if and only if the division ring D satisfies the rational identity.

Pappus's hexagon theorem holds in a desarguesian projective plane {displaystyle mathbb {P} _{2}D} if and only if D is a field; it corresponds to the identity {displaystyle forall a,bin D,quad acdot b=bcdot a} . Fano's axiom (which states a certain intersection does not happen) holds in {displaystyle mathbb {P} _{2}D} if and only if D has characteristic {stile di visualizzazione neq 2} ; it corresponds to the identity a + a = 0. References Rowen, Louis Halle, ed. (1980). Polynomial Identities in Ring Theory. Matematica pura e applicata. vol. 84. Stampa accademica. doi:10.1016/s0079-8169(08)x6032-5. ISBN 9780125998505. Amitsur, S. UN. (1966). "Rational Identities and Applications to Algebra and Geometry". Giornale di algebra. 3 (3): 304–359. doi:10.1016/0021-8693(66)90004-4. Categorie: Incidence geometryTheorems in projective geometry

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