Abstract
Let N be the number of triangles in an Erdős–Rényi graph \({\mathcal {G}}(n,p)\) on n vertices with edge density \(p=d/n,\) where \(d>0\) is a fixed constant. It is well known that N weakly converges to the Poisson distribution with mean \({d^3}/{6}\) as \(n\rightarrow \infty \). We address the upper tail problem for N, namely, we investigate how fast k must grow, so that \({\mathbb {P}}(N\ge k)\) is not well approximated anymore by the tail of the corresponding Poisson variable. Proving that the tail exhibits a sharp phase transition, we essentially show that the upper tail is governed by Poisson behavior only when \(k^{1/3} \log k< (\frac{3}{\sqrt{2}} - {o(1)})^{2/3} \log n\) (sub-critical regime) as well as pin down the tail behavior when \(k^{1/3} \log k> (\frac{3}{\sqrt{2}} + {o(1)})^{2/3} \log n\) (super-critical regime). We further prove a structure theorem, showing that the sub-critical upper tail behavior is dictated by the appearance of almost k vertex-disjoint triangles whereas in the supercritical regime, the excess triangles arise from a clique like structure of size approximately \((6k)^{1/3}\). This settles the long-standing upper-tail problem in this case, answering a question of Aldous, complementing a long sequence of works, spanning multiple decades and culminating in Harel et al. (Duke Math J 171(10):2089–2192, 2022), which analyzed the problem only in the regime \(p\gg \frac{1}{n}.\) The proofs rely on several novel graph theoretical results which could have other applications.
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Notes
\(\approx \) will be informally used throughout the article to denote ‘close to’ in a sense whose interpretation might change across locations depending on the context. This will not feature in any proof and hence we will refrain from being more precise.
We write \(f \lesssim g\) to denote \(f = O(g)\); \(f \asymp g\) means \(f = \Theta (g)\); \(f \sim g\) means \(f = (1+o(1))g\) and \(f \ll g\) means \(f = o(g)\).
We say that H is the \(\textsf{TISG}\) spanned by \(\ell \) triangles \(T_1,\ldots ,T_\ell ,\) if H is connected and is the union of \(T_1,\ldots ,T_\ell \).
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Acknowledgements
S.G. thanks Sourav Chatterjee for mentioning to him the question of studying the upper tail of N, which he had learnt from David Aldous. S.G. also thanks Noga Alon and Wojciech Samotij for useful discussions and in particular for pointing him to the earlier appearances of versions of Proposition 3.7 in [23] and [18]. His research was partially supported by NSF grant DMS-1855688, NSF CAREER grant DMS-1945172, and a Sloan Fellowship. E.H. was partially supported by NSF grant DMS-1855688. K.N was supported by the National Research Foundation of Korea (NRF-2019R1A5A1028324, NRF-2019R1A6A1A10073887).
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Ganguly, S., Hiesmayr, E. & Nam, K. Upper Tail Behavior of the Number of Triangles in Random Graphs with Constant Average Degree. Combinatorica (2024). https://doi.org/10.1007/s00493-024-00086-3
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DOI: https://doi.org/10.1007/s00493-024-00086-3