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A graph with four special vertices (called terminals) is wye-delta reducible if we can obtain a graph on four vertices by a sequence of wye-delta and delta-wye operations and series-parallel reductions, none of which is allowed to remove any of the terminals. A good characterization of wye-delta reducible 3-connected planar graphs with four terminals is given. The proofs yield an O(n2) time algorithm that either exhibits an obstruction to the 4-terminal reducibility or returns a sequence of wye-delta operations and series-parallel reductions that reduce the input graph to a subgraph of K4 whose vertices are the terminals.  We also discuss terminal wye-delta reducibility when a mixture of vertices and faces are treated as terminals. It is also shown that a sufficiently connected cubic graph is wye-delta reducible if and only if it does not contain the Petersen graph as a minor.  The main ingredient in the proofs is a good characterization of planar graphs with four terminals that do not admit a rooted K2,4 minor with the four terminals corresponding to the roots on the large side of the bipartition of K2,4. Up to small connectivity reductions, cases without the rooted minor fall into five structural cases that lead to a polynomial-time algorithm for recognition of these graphs and construction of rooted K2,4 minors. This result is of independent interest in structural graph theory.

Four terminal planar Delta-Wye reducibility via rooted K2,4 minors

Four terminal planar Delta-Wye reducibility via rooted K_2,4 minors