Regions of totally real arrangements of rational curves
Abstract
We prove a plane-curve analogue of Zaslavsky's theorem for arrangements of real rational curves for which every complex singularity with more than one local branch is real and all of its branches are real. If at least one component has odd degree, the number of real regions equals the total rational Betti number of the complexified complement. We also give a formula for bounded affine regions which records the number of branches at infinity and the connected components of the real affine curve. Oriented regions generate the middle Borel--Moore homology over ℚ, and they generate it over ℤ when the component degrees are coprime. The relations are described by the circles in the real normalization and, in an affine chart, by winding functions. These results imply equality of combinatorial rank and real combinatorial rank in the framework of Brown--Dupont and Sturmfels--Telen. Examples include singular rational cubics, arrangements of conics, Chebyshev curves, and the plane model of the $27$ lines on a split cubic surface.
Related
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BibTeX
@misc{yoshinaga2026regions,
title = {{Regions of totally real arrangements of rational curves}},
author = {Masahiko Yoshinaga},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.00861},
eprint = {2611.00861},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.00861},
}
RIS
TY - UNPB
TI - Regions of totally real arrangements of rational curves
AU - Masahiko Yoshinaga
PY - 2026
DA - 2026/11/01
PB - arXiv
DO - 10.48550/arxiv.2611.00861
UR - https://arxiv.org/abs/2611.00861
AB - We prove a plane-curve analogue of Zaslavsky's theorem for arrangements of real rational curves for which every complex singularity with more than one local branch is real and all of its branches are real. If at least one component has odd degree, the number of real regions equals the total rational Betti number of the complexified complement. We also give a formula for bounded affine regions which records the number of branches at infinity and the connected components of the real affine curve. Oriented regions generate the middle Borel--Moore homology over ℚ, and they generate it over ℤ when the component degrees are coprime. The relations are described by the circles in the real normalization and, in an affine chart, by winding functions. These results imply equality of combinatorial rank and real combinatorial rank in the framework of Brown--Dupont and Sturmfels--Telen. Examples include singular rational cubics, arrangements of conics, Chebyshev curves, and the plane model of the $27$ lines on a split cubic surface.
ER -