The extremal spectral radii of k -uniform supertrees

In this paper, we study some extremal problems of three kinds of spectral radii of k-uniform hypergraphs (the adjacency spectral radius, the signless Laplacian spectral radius and the incidence Q-spectral radius). We call a connected and acyclic k-uniform hypergraph a supertree. We introduce the operation of “moving edges” for hypergraphs, together with the two special cases of this operation: the edge-releasing operation and the total grafting operation. By studying the perturbation of these kinds of spectral radii of hypergraphs under these operations, we prove that for all these three kinds of spectral radii, the hyperstar Sn,k attains uniquely the maximum spectral radius among all k-uniform supertrees on n vertices. We also determine the unique k-uniform supertree on n vertices with the second largest spectral radius (for these three kinds of spectral radii). We also prove that for all these three kinds of spectral radii, the loose path Pn,k attains uniquely the minimum spectral radius among all k-th power hypertrees of n vertices. Some bounds on the incidence Q-spectral radius are given. The relation between the incidence Q-spectral radius and the spectral radius of the matrix product of the incidence matrix and its transpose is discussed. B Jia-Yu Shao [email protected] Honghai Li [email protected] Liqun Qi [email protected] 1 College of Mathematics and Information Science, Jiangxi Normal University, Nanchang 330022, China 2 Department of Mathematics, Tongji University, Shanghai, China 3 Department of Applied Mathematics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 123 Author's personal copy