Hassett's weighted blow-up conjecture for unordered points on the projective line

Let nn be a positive integer and let 1n+ϵ\frac{1}{n}+\frac{}{}\epsilon be the common weight on 2n2n marked points, with the weighted moduli space M0,(1n+ϵ,,1n+ϵ)\overline{\mathcal{M}}_{0,(\frac{1}{n}+\epsilon,\ldots,\frac{1}{n}+\epsilon)}. Let M~0,2n\widetilde{M}_{0,2n} be the coarse moduli space of

[M0,(1n+ϵ,,1n+ϵ)/S2n],[\overline{\mathcal{M}}_{0,(\frac{1}{n}+\epsilon,\ldots,\frac{1}{n}+\epsilon)}/S_{2n}],

where S2nS_{2n} permutes the marked points, and let

P(H0(P1,OP1(2n)))/ ⁣ ⁣/PGL2\mathbb{P}(\operatorname{H}^0(\mathbb{P}^1,\mathcal{O}_{\mathbb{P}^1}(2n)))/\!\!/\operatorname{PGL}_2

be the GIT moduli space of 2n2n unordered points on P1\mathbb{P}^1, linearized via O(2)\mathcal{O}(2). Hassett's conjecture. There is a map

M~0,2nP(H0(P1,OP1(2n)))/ ⁣ ⁣/PGL2\widetilde{M}_{0,2n}\to \mathbb{P}(\operatorname{H}^0(\mathbb{P}^1,\mathcal{O}_{\mathbb{P}^1}(2n)))/\!\!/\operatorname{PGL}_2

which is a weighted blow-up. The conjecture identifies the birational geometry of the moduli space of unordered weighted stable curves with the GIT compactification of unordered points on P1\mathbb{P}^1; the stated result is proven by showing the corresponding statement for ordered points and constructing the induced map.

Sources & referencesView supporting material

Primary source

Andrea Di Lorenzo and Giovanni Inchiostro, “Stable maps to quotient stacks with a properly stable point”, arXiv:2411.16141 (2025).

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