The neighborhood of a vertex in a graph consists of the vertex
itself, together with all vertices that are connected to it by an
edge. Each graph has a variable xi associated with the i-th vertex,
and the vertex has a known value that is equal to the sum of the
variables for all neighborhood vertices. Start with a graph with 5
vertices forming a pentagon, with edges joining vertices 1 and 2, 2
and 3, 3 and 4, 4...