The obvious answer is always overlooked.
Known as Whitehead’s Law.
Abstract
We continue our quest for real enumerative invariants not sensitive to changing the real structure and extend the construction we uncovered previously for counting curves of anti-canonical degree \(\leqslant 2\) on del Pezzo surfaces with \(K^2=1\) to curves of any anti-canonical degree and on any del Pezzo surfaces of degree \(K^2\leqslant 3\).
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Notes
In fact, this theorem, with appropriate definitions for the numbers \(\Gamma _{B,k}\), holds for all real rational surfaces X with any \({\text {Pin}}^-\)-structure on \(X_\mathbb R\).
Properties (4) and (5) hold without any assumption on a real structure of X.
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Acknowledgements
The idea of this work arose during a stay of the second author at the Max-Planck Institute for Mathematics in Bonn in spring 2021 and took its shape during a RIP-stay of the authors at the Mathematisches Forschungsinstitut Oberwolfach in summer 2021. We thank the both institutions for hospitality and excellent (despite a complicated pandemic situation) working conditions. Our special thanks go to R. Rasdeaconu, discussions with whom were among the motivations for this study, and to J. Solomon, for encouragement and helpful remarks. We also thankful to the referee for careful reading and valuable comments. The second author was partially supported by the grant ANR-18-CE40-0009 of French Agence Nationale de Recherche and the grant by Ministry of Science and Higher Education of Russia under the contract 075-15-2019-1620 with St. Petersburg Department of Steklov Mathematical Institute.
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Finashin, S., Kharlamov, V. On wall-crossing invariance of certain sums of Welschinger numbers. Sel. Math. New Ser. 29, 41 (2023). https://doi.org/10.1007/s00029-023-00838-x
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DOI: https://doi.org/10.1007/s00029-023-00838-x