Laplacian Spectral Indices of Hexacyclic Systems

The hexacyclic system graph Fn is the graph derived from a linear hexagonal chain Ln with n > 1 hexagons by identifying two pairs of ends of Ln. The M¨obious hexacyclic system graph Mn is the graph derived from a linear hexagonal chain Ln with n > 1 hexagons by identifying two pairs of ends of Ln with a twist. In this paper, we compute, in a closed form, the resolvent energy, the Laplacian and the signless Laplacian resolvent energy, as well as the resolvent Estrada index and the resolvent signless Estrada index of Fn and Mn. All five indices are expressed as a rational function in the number n of hexagons, defined in terms of Chebyshev polynomials of the first and the second kind. Those expressions allow for a fast numerical computation of indices and for deducing sharp bounds on their growth.