Non-simple telltale intersection-multiplicity conjecture

Let RP1R\to\mathbb P^1 be a rational elliptic surface, let LL determine a Severi curve with normalization V~(L)\widetilde{V}(L), and let fL:V~(L)Sym2(P1)f_L:\widetilde{V}(L)\to\operatorname{Sym}^2(\mathbb P^1) record the two branch points of a rational bisection. Let ΔSym2(P1)\Delta\subset\operatorname{Sym}^2(\mathbb P^1) be the diagonal conic, and let s1+s2+mNs_1+s_2+mN be a non-simple telltale, where s1,s2s_1,s_2 are sections, mm is a positive integer, and NN is a nodal fiber. Write σ1(m)\sigma_1(m) for the sum of the positive divisors of mm. Non-simple telltale multiplicity conjecture. The intersection multiplicity of fLf_L with Δ\Delta at s1+s2+mNs_1+s_2+mN is

{2σ1(m),s1s2,σ1(m),s1=s2, m is not a square,σ1(m)1,s1=s2, m is a square.\begin{cases} 2\sigma_1(m), & s_1\ne s_2,\\ \sigma_1(m), & s_1=s_2,\ m\text{ is not a square},\\ \sigma_1(m)-1, & s_1=s_2,\ m\text{ is a square}. \end{cases}

Together with the transverse contribution from simple telltales, this conjecture would control the intersection number governing the degree calculation for the Severi curves. Its status is not resolved in the supplied material.

Sources & referencesView supporting material

Primary source

François Greer, Joseph Helfer and John Sheridan, “Severi curves of rational elliptic surfaces”, arXiv:2306.03200 (2025).

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