Nonattainment conjecture for the rectilinear crossing density of the complete graphon

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Let W1W^1 denote the graphon of the complete graph, let F\mathcal F be the family of graphon drawings used to define the rectilinear crossing density, and let cd‾(W,f)\overline{\mathop{\mathrm{cd}}}(W,f) be the corresponding density for f∈Ff\in\mathcal F. Write cd‾(W1,D)\overline{\mathop{\mathrm{cd}}}(W^1,\mathcal D) for the infimum over the admissible drawing family D\mathcal D.

Nonattainment conjecture. There exists no f∈Ff\in\mathcal F such that

cd‾(W1,D)=cd‾(W1,f).\overline{\mathop{\mathrm{cd}}}(W^1,\mathcal D)=\overline{\mathop{\mathrm{cd}}}(W^1,f).

In particular, the infimum in

lim⁡n→∞cr‾(Kn)(n4)=inf⁡Rc(λR)\lim_{n\rightarrow\infty}\frac{\overline{\mathop{\mathrm{cr}}}(K_n)}{\binom{n}{4}}=\inf_{R}c(\lambda_R)

can not be substituted for a minimum. This concerns whether the limiting rectilinear crossing density of complete graphs is attained by a single admissible graphon drawing, and the source presents the attainment question as unresolved.

References

Primary source

Oriol Solé-Pi, “An algorithm for estimating the crossing number of dense graphs, and continuous analogs of the crossing and rectilinear crossing numbers”, arXiv:2401.00665 (2025).

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