The node-count conjecture for factoriality of nodal threefolds

Let XP(14,r)X\subset\mathbb{P}(1^{4},r) be the nodal double-cover threefold described above, and let VP4V\subset\mathbb{P}^{4} be a nodal hypersurface of degree nn as described above. The notation Sing(X)\operatorname{Sing}(X) and Sing(V)\operatorname{Sing}(V) denotes their singular loci, and a threefold is Q\mathbb{Q}-factorial if every Weil divisor has a positive multiple that is Cartier.

Node-count conjecture. The inequalities

Sing(X)<(2r1)r|\operatorname{Sing}(X)|<(2r-1)r

and

Sing(V)<(n1)2|\operatorname{Sing}(V)|<(n-1)^{2}

respectively imply the Q\mathbb{Q}-factoriality of the threefolds XX and VV.

The claim proposes that the examples immediately preceding it are asymptotically sharp: at the displayed node counts, the corresponding threefolds can fail to be Q\mathbb{Q}-factorial, while strict improvement of those bounds should force Q\mathbb{Q}-factoriality. The supplied text gives no resolution of this expectation.

Sources & referencesView supporting material

Primary source

Ivan Cheltsov, “On factoriality of nodal threefolds”, arXiv:math/0405221 (2004).

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