Symmetry, Integrability and Geometry: Methods and Applications (SIGMA)

SIGMA 21 (2025), 012, 50 pages      arXiv:2312.10759      https://doi.org/10.3842/SIGMA.2025.012

Counting Curves with Tangencies

Indranil Biswas a, Apratim Choudhury b, Ritwik Mukherjee c and Anantadulal Paul d
a) Department of Mathematics, Shiv Nadar University, NH91, Tehsil Dadri, Greater Noida, Uttar Pradesh 201314, India
b) Institut für Mathematik, Humboldt-Universität zu Berlin, Unter den Linden 6, Berlin 10099, Germany
c) School of Mathematical Sciences, National Institute of Science Education and Research, Bhubaneswar, An OCC of Homi Bhabha National Institute, Khurda 752050, Odisha, India
d) International Center for Theoretical Sciences, Survey No. 151, Hesaraghatta, Uttarahalli Hobli, Sivakote, Bangalore 560089, India

Received May 04, 2024, in final form February 07, 2025; Published online February 23, 2025

Abstract
Interpreting tangency as a limit of two transverse intersections, we obtain a concrete formula to enumerate smooth degree $d$ plane curves tangent to a given line at multiple points with arbitrary order of tangency. Extending that idea, we then enumerate curves with one node with multiple tangencies to a given line of any order. Subsequently, we enumerate curves with one cusp, that are tangent to first order to a given line at multiple points. We also present a new way to enumerate curves with one node; it is interpreted as a degeneration of a curve tangent to a given line. That method is extended to enumerate curves with two nodes, and also curves with one tacnode are enumerated. In the final part of the paper, it is shown how this idea can be applied in the setting of stable maps and perform a concrete computation to enumerate rational curves with first-order tangency. A large number of low degree cases have been worked out explicitly.

Key words: enumeration of curves; tangency; nodal curve; cusp.

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