Spectral Turán problems for intersecting even cycles

Abstract

Let $C_{2k_1,2k_2,\dots ,2k_t}$ denote the graph obtained by intersecting $t$ distinct even cycles $C_{2k_1},C_{2k_2},\dots ,C_{2k_t}$ at a unique vertex. In this paper, we determine the unique graphs with maximum adjacency spectral radius among all graphs on $n$ vertices that do not contain any $C_{2k_1,2k_2,\dots ,2k_t}$ as a subgraph, for $n$ sufficiently large. When one of the constituent even cycles is a $C_4$, our results improve upper bounds on the Turán numbers for intersecting even cycles that follow from more general results of Füredi [20] and Alon, Krivelevich and Sudakov [1]. Our results may be seen as extensions of previous results for spectral Turán problems on forbidden even cycles $C_{2k},k\ge 2$ (see [8, 34, 44, 45]).