Codimension formula for Severi varieties of unicuspidal rational curves

Let r=(r1,,rn)N>0n{\bf r}=(r_1,\ldots,r_n)\in \mathbb{N}_{>0}^n, and let VrMd,g;Sn\mathcal{V}_{\bf r}\subset M^n_{d,g;\mathrm{S}} be the subvariety of maps f:P1Pnf:\mathbb{P}^1\rightarrow\mathbb{P}^n with a unique cuspidal singularity having semigroup S\mathrm{S} and ramification profile r{\bf r}. Assume d=deg(f)max(n,2g2)d=\deg(f)\geq\max(n,2g-2). For sSs\in\mathrm{S}, let ρ(s)\rho(s) be the number of elements of NS\mathbb{N}\setminus\mathrm{S} strictly greater than ss. Codimension conjecture.

cod(Vr,Mdn)=i=1n(rii)+sBϕ(s)ρ(s)srρ(s)1.\operatorname{cod}(\mathcal{V}_{\bf r},M^n_d)=\sum_{i=1}^n(r_i-i)+\sum_{s\in B}\phi(s)\rho(s)-\sum_{s\in{\bf r}^{\bullet}}\rho(s)-1.

Here BB, ϕ\phi, and r{\bf r}^{\bullet} are the combinatorial quantities defined from the semigroup and ramification profile in the preceding setup. The formula predicts the codimension of the corresponding Severi variety, accounting for both ramification conditions and the additional conditions imposed by Betti elements. The supplied text gives no evidence that the conjecture has been resolved.

Sources & referencesView supporting material

Primary source

Ethan Cotterill and Renato Vidal Martins, “Towards Brill–Noether theory for cuspidal curves”, arXiv:2302.13993 (2023).

Additional references

2 papers in this index state this conjecture (2020–2023). The statement above is taken from the most recent of them; the others are arXiv:2006.09580.

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