2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/114992We introduce the matrix sums that represent a discrete analog of the matrix models with quartic potential. The probability space is given by the set of all simple n-vertex graphs with the Gibbs weight determined by the graph Laplacian. We study the large-n limit of the free energy per site and show that it is determined by the number of connected acyclic diagrams on the set of two-valent vertices.18 pages, 3 figures; misprints corrected, minor improvements of the text, one reference addedMathematical PhysicsCombinatorics05C80;15A52;60F99text