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

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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 M‾0,(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

[M‾0,(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(H⁡0(P1,OP1(2n)))/ ⁣ ⁣/PGL⁡2\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~0,2n→P(H⁡0(P1,OP1(2n)))/ ⁣ ⁣/PGL⁡2\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.

References

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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