Abstract
The crossing number of a graph G, denoted by cr(G), is defined as the smallest possible number of edge-crossings in a drawing of G in the plane. The cone CG, obtained from G by adding a new vertex adjacent to all the vertices in G. Let \(\phi _\mathrm {s}(k)=\min \{cr(CG)-cr(G)\}\), where the minimum is taken over all the simple graphs G with crossing number k. Alfaro et al. (SIAM J Discrete Math, 32: 2080–2093, 2018) determined \(\phi _\mathrm {s}(k)\le k\) for \(k\ge 3\), and proved \(\phi _\mathrm {s}(3)=3\), \(\phi _\mathrm {s}(4)=4\) and \(\phi _\mathrm {s}(5)=5\). In this work, we improve the upper bound for \(\phi _\mathrm {s}(k)\) and our main result includes the following, slightly surprising, fact: \(\phi _\mathrm {s}(6)=5\) and \(\phi _\mathrm {s}(7)=6\).
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The authors are very grateful to the anonymous referees for many comments and suggestions, which are very helpful to improve the presentation of this paper.
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The work was supported by the National Natural Science Foundation of China (No. 11871206), Hunan Provincial Natural Science Foundation of China (No. 2020JJ5095) and Scientific Research Fund of Hunan Provincial Education Department (No. 19B116).
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Ding, Z., Huang, Y. A Note on the Crossing Number of the Cone of a Graph. Graphs and Combinatorics 37, 2351–2363 (2021). https://doi.org/10.1007/s00373-021-02361-2
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DOI: https://doi.org/10.1007/s00373-021-02361-2