The upper-tail exponent conjecture for cliques

Let HH be a graph, let G(n,p)G(n,p) be the binomial random graph, and let d4cdHd4cd_H denote the number of copies of HH in G(n,p)G(n,p). Write vKv_K and eKe_K for the numbers of vertices and edges of a subgraph KHK\subseteq H, set

Ψ(K,n,p)=nvKpeK,\Psi(K,n,p)=n^{v_K}p^{e_K},

and let mHm_H and ΔH\Delta_H denote the relevant graph-density and maximum-degree parameters. Define

MH(n,p)={n2pΔH,pn1/ΔH,minKH(Ψ(K,n,p))1/αK,n1/mHpn1/ΔH,M_H(n,p)=\begin{cases} n^2p^{\Delta_H},&p\geq n^{-1/\Delta_H},\\ \displaystyle\min_{K\subseteq H}\bigl(\Psi(K,n,p)\bigr)^{1/\alpha_K^*},&n^{-1/m_H}\leq p\leq n^{-1/\Delta_H}, \end{cases}

where αK\alpha_K^* is the fractional independence number of KK.

Upper-tail exponent conjecture. For any HH and p>n1/mHp>n^{-1/m_H},

Pr(ξH2EξH)=exp[ΘH(min{minKH, eK>0Ψ(K,n,p),MH(n,p)t})].\Pr\left(\xi_H\geq 2{\bf E}\xi_H\right)=\exp\left[-\Theta_H\left(\min\left\{\min_{K\subseteq H,\ e_K>0}\Psi(K,n,p),M_H(n,p)t\right\}\right)\right].

Here tt is the parameter governing the upper-tail scale in the paper. The conjecture proposes that the true exponent is determined, up to constants depending on HH, by the largest of the previously established lower-bound mechanisms; the asserted formula remains unproved in general.

Sources & referencesView supporting material

Primary source

Bobby DeMarco and Jeff Kahn, “Upper Tails for Cliques”, arXiv:1111.6687 (2012).

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