Bipartite graphs and eigenvalue problems

I will tell you how to build matrix models using graphs on a Riemann surface and how various identities with differential operators are obtained from them. In particular, we obtain the Mironov-Morozov-Natanzon cut-and-join equation, which describes the merging of branching points. (А section in joint work with S.M.Natanzon, TMP 2020)