  We prove a formula for a characteristic polynomial of an operator expressed
as a polynomial of rank 1 operators. The formula uses a discrete analog of path
integration and implies a generalization of the Forman-Kenyon's formula [4,6]
for a determinant of the graph Laplacian (which, in its turn, implies the
famous matrix-tree theorem by Kirchhoff) as well as its level 2 analog, where
the summation is performed over triangulated nodal surfaces with boundary.
