Assume that u(p) > u(q). A subgraph H of G is constructedin stages as follows. Let H0 be the node p. Next let H1 consist of allnodes r and edges rp in G, such that r is a neighbor of p and u(r) > u(p).Inductively, having defined Hj , let Hj+1 consist of all edges in Hj and allnodes s and edges st in G such that t ∈ Hj and u(s) > u(t). This gives anincreasing sequences of subnetworks Eventually no new edges are added and the process ends. Let H be theunion of the Hj . By restricting the conductivity function γ to the edgesin the subgraph H, (H, γ) may be considered a resistor network. For eachnode r in H, let ψ(r) be the (algebraic) sum of the currents from r to its neighboring nodes in H, and let φ(r) be the (algebraic) sum of the currentsfrom r to its neighboring nodes in G. By the construction of H, ψ(r) ≤ φ(r)for all nodes r in H. For all nodes r in H which are interior nodes of G,ψ(r) ≤ 0, so the function ψ(r) can be positive only at a node of H which isa boundary node of G. Then