Making Multicurves Cross Minimally on Surfaces
<|reference_start|>Making Multicurves Cross Minimally on Surfaces: On an orientable surface $S$, consider a collection $\Gamma$ of closed curves. The (geometric) intersection number $i_S(\Gamma)$ is the minimum number of self-intersections that a collection $\Gamma'$ can have, where $\Gamma'$ results from a continuous deformation (homotopy) of $\Gamma$. We provide algorithms that compute $i_S(\Gamma)$ and such a $\Gamma'$, assuming that $\Gamma$ is given by a collection of closed walks of length $n$ in a graph $M$ cellularly embedded on $S$, in $O(n \log n)$ time when $M$ and $S$ are fixed.   The state of the art is a paper of Despr\'e and Lazarus [SoCG 2017, J. ACM 2019], who compute $i_S(\Gamma)$ in $O(n^2)$ time, and $\Gamma'$ in $O(n^4)$ time if $\Gamma$ is a single closed curve. Our result is more general since we can put an arbitrary number of closed curves in minimal position. Also, our algorithms are quasi-linear in $n$ instead of quadratic and quartic, and our proofs are simpler and shorter.   We use techniques from two-dimensional topology and from the theory of hyperbolic surfaces. Most notably, we prove a new property of the reducing triangulations introduced by Colin de Verdi\`ere, Despr\'e, and Dubois [SODA 2024], reducing our problem to the case of surfaces with boundary. As a key subroutine, we rely on an algorithm of Fulek and T\'oth [JCO 2020].<|reference_end|>
@article{dubois2024making,
    title={Making Multicurves Cross Minimally on Surfaces},
    author={Lo"ic Dubois},
    journal={arXiv preprint arXiv:2402.04789},
    year={2024},
    archivePrefix={arXiv},
    eprint={2402.04789},
    primaryClass={cs.CG}
}