ar X iv : m at h / 07 01 88 5 v 1 [ m at h . A G ] 3 0 Ja n 20 07 CONIC - CONNECTED MANIFOLDS
نویسندگان
چکیده
We study a particular class of rationally connected manifolds, X ⊂ P , such that two general points x, x′ ∈ X may be joined by a conic contained in X. We prove that these manifolds are Fano, with b2 6 2. Moreover, a precise classification is obtained for b2 = 2. Complete intersections of high dimension with respect to their multi-degree provide examples for the case b2 = 1. The proof of the classification result uses a general characterization of rationality, in terms of suitable covering families of rational curves. A Gaetano Scorza che un secolo fa aveva colto l’importanza delle varietà razionalmente connesse considerando la classe particolare di varietà conicamente connesse formata da quelle di ultima specie. ”Invece per le V4 di prima e terza specie arrivo a caratterizzarle tutte valendomi della teoria dei sistemi lineari sopra una varietà algebrica e, per le ultime, della circostanza che esse contengono un sistema ∞ di coniche cosı̀ che per ogni loro coppia di punti passa una e una sola conica. Inoltre la natura dei ragionamenti è tale da mostrare come i risultati ottenuti possano estendersi, almeno per la maggior parte, alle varietà (di prima e ultima specie) a un numero qualunque di dimensioni, ...”, [Sc2, Opere Scelte, vol. 1, p. 253].
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