More on Rectangles Tiled by Rectangles
David G. Mead, Sherman K. Stein · American Mathematical Monthly · 1993
1. THE METHOD. Let G be a connected linear graph with m vertices and n edges, e1, e2, ..., en, such that each edge is incident to two distinct vertices. There may be more than one edge incident to the same vertices. If edge ei is incident to the vertices A and B we orient ei by selecting one of the orientations AB or BA. (We may think of the orientation AB as an arrow from A to B and the algebraic boundary of ei as B A.). Let C1 consist of the formal sums E7 lxiei, where xi is real. Such a sum is shorthand for a function h from the set of edges to the real numbers, where