Counting curves with tangencies

Indranil Biswas, Apratim Choudhury, Ritwik Mukherjee, Anantadulal Paul

Published 2023-12-17, updated 2025-02-11Version 3

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. One nodal curves with multiple tangencies of any order are enumerated. Also, one cuspidal curves, that are tangent to first order to a given line at multiple points, are enumerated. 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.