A sufficient condition for nonsingularity of two-dimensional finite-gap Schrödinger operator discretization of a quad graph

We construct a two-dimensional Schr\"{o}dinger operator on a quad-graph using finite-gap approach. A guad-graph is a planar graph with only 4-sided faces. We define the non-singularity of the operator as the positivity of all of his coefficients. At last, we prove some sufficient conditions of such positivity on the spectral curve and the graph.