Note on the sum of the smallest and largest eigenvalues of a triangle-free graph

Let $G$ be a triangle-free graph on $n$ vertices with adjacency matrix eigenvalues $μ_1(G)\geq μ_2(G)\geq \dots \geq μ_n(G)$. In this paper we study the quantity $μ_1(G)+μ_n(G).$ We prove that for any triangle-free graph $G$ we have $μ_1(G)+μ_n(G)\leq (3-2\sqrt{2})n.$ This was proved for regular graphs by Brandt, we show that the condition on regularity is not necessary. We also prove that among triangle-free strongly regular graphs the Higman-Sims graph achieves the maximum of $\frac{μ_1(G)+μ_n(G)}{n}.$

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