A connected plane graph G with two distinguished vertices can be reduced to a single edge between these two vertices using certain local transformations, including series/parallel reductions and wye-delta and delta-wye transformations. Moreover a reduction consisting of these transformations can be found algorithmically in polynomial time. It follows that if an invariant of plane graphs is known to be #P-hard to compute then the invariant must be fundamentally incompatible with these local transformations in some way. We discuss two examples: polynomial invariants of knots and links and the reliability of plane networks.
Title
A note on delta-wye-delta reductions of plane graphs