Counting color-critical subgraphs under Nikiforov's condition
Longfei Fang, Huiqiu Lin, Mingqing Zhai
Abstract
For a graph $G$ with $m$ edges, let $ρ(G)$ be its spectral radius, and let $N_F(G)$ denote the number of copies of $F$ in $G$. Nikiforov [Combin. Probab.\,Comput., 2002] proved that for $r\geq 2$, if $ρ(G)>\sqrt{(1-1/r)2m}$, then $N_{K_{r+1}}(G)\geq 1$. Furthermore, Bollobás and Nikiforov [J. Combin. Theory, Ser. B, 2007] used $ρ(G)$ to establish a counting inequality for complete subgraphs. In this paper, we generalize and strengthen the above results to any color-critical graph $F$ with chromatic number at least four. More precisely, we demonstrated that under Nikiforov's condition, the number of copies of $F$ in $G$ satisfies $N_F(G)\geq\big(γ_F-o(1)\big)m^{(|F|-2)/2},$ where both the leading item and the constant $γ_F$ are optimal. Let $F$ be a non-star graph with $χ(F)=r+1$, and let $G$ be any graph of sufficiently large size $m$ satisfying $N_F(G)=o(m^{|F|/2})$. To support the aforementioned counting arguments, we initially employ the method of progressive induction to tackle spectral problems, proving that $ρ(G)\leq\sqrt{(1-1/r+o(1))2m}$ for $r\geq 3$, and $ρ(G)\leq\sqrt{(1+o(1))m}$ for $r\in \{1,2\}$. Furthermore, we establish a stability result for edge-spectral supersaturation: specifically, if $r\geq 3$ and $ρ(G)\geq\sqrt{(1-1/r-o(1))2m}$, then $G$ differs from an $r$-partite Turán graph by $o(m)$ edges; if $r\in \{1,2\}$ and $ρ(G)\geq\sqrt{(1-o(1))m}$, then $G$ differs from a complete bipartite graph by $o(m)$ edges. This implies the well-known Erdos-Simonovits stability theorem and existing spectral stability theorems, by strengthening the setting from $F$-free graphs to graphs containing only a limited number of copies of $F$. Finally, we propose several counting-related open problems for further investigation.