Graphs with the Reduced Spectrum in the Unit Interval


Aleksandar Torgašev


By the reduced spectrum (r.s.) of a finite connected graph, we mean the set of all its eigenvalues with the maximal and the minimal eigenvalues excluded. In this paper we characterize all finite connected graphs having at least one positive and at least one negative eigenvalue in their reduced spectrum, whose r.s. lies in the unit interval $[-1,1)$.