Scaling conjecture for longest subsequences in uniform separable permutations and cographs

Let calpha(1/2)calpha(1/2) and X(1/2)X(1/2) be the exponent and limiting random variable from the Brownian separable permuton result. Let coverlineσncoverline{\sigma}_n and coverlineGncoverline{G}_n be, respectively, a uniform separable permutation and a uniform separable cograph of size nn.

Uniform separable scaling conjecture. There is a deterministic constant c>0c>0 such that

LIS(σn)nα(1/2)ndcX(1/2)\frac{\operatorname{LIS}(\overline{\sigma}_n)}{n^{\alpha(1/2)}} \xrightarrow[n\rightarrow \infty]{\mathrm{d}} c \cdot X(1/2)

and

LIN(Gn)nα(1/2)ndcX(1/2).\frac{\operatorname{LIN}(\overline{G}_n)}{n^{\alpha(1/2)}} \xrightarrow[n\rightarrow \infty]{\mathrm{d}} c\cdot X(1/2).

The conjecture extends the Brownian separable permuton and cograph results to uniform models; numerical simulations suggest c0.901c\approx 0.901, and the source notes that this is more precise than an earlier conjecture.

Sources & referencesView supporting material

Primary source

Arka Adhikari, Jacopo Borga, Thomas Budzinski, William Da Silva and Delphin Sénizergues, “The longest increasing subsequence of Brownian separable permutons”, arXiv:2506.19123 (2025).

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