A zero-part formula for logarithmic double ramification intersections

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Let dd be the degree, let ν\boldsymbol{\nu} be the ramification data, and let logDR⁡0(d,−ν)\operatorname{logDR}_0(d,-\boldsymbol{\nu}) denote the logarithmic double ramification cycle. Let ψ1\psi_1 be the cotangent-line class at the first marking and let branch⁡m−1−e\operatorname{branch}_{m-1-e} be the corresponding branch class. Zero-part formula. After inserting ν1=0\nu_1=0,

∫logDR⁡0(d,−ν)ψ1e⋅branch⁡m−1−e∣ν1=0=(m−1−e)(m−2)!(e+1)!⋅(∑i=1e+1∏j=0m−e−3(d−(i+j)k2)).\int_{\operatorname{logDR}_0(d,-\boldsymbol{\nu})} \psi_1^{e}\cdot \operatorname{branch}_{m-1-e}\big|_{\nu_1=0} =(m-1-e)\frac{(m-2)!}{(e+1)!} \cdot \Big( \sum_{i=1}^{e+1} \prod_{j=0}^{m-e-3}(d-(i+j)\frac{k}{2})\Big).

The formula is introduced as a conjectural expression obtained by inserting a contracted marked point into the recursion. The text notes that it currently has no geometric meaning in this form, while suggesting that it should acquire one.

References

Primary source

Renzo Cavalieri, Hannah Markwig and Johannes Schmitt, “One part leaky covers”, arXiv:2509.04335 (2025).

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