2-restricted extensions of partial embeddings of graphs

Let K be a subgraph of G. Suppose that we have a 2-cell embedding of K in some surface and that for each K-bridge in G, one or two simple embeddings in faces of K are prescribed. Obstructions for existence of extensions of the embedding of K to an embedding of G are studied. It is shown that minimal obstructions possess certain combinatorial structure that can be described in an algebraic way by means of forcing chains of K-bridges. The geometric structure of minimal obstructions is also described. It is shown that they have “millipede” structure that was observed earlier in some special cases (disc, Möbius band). As a consequence it is proved that if one is allowed to reroute the branches of K, one can obtain a subgraph K′ of G homeomorphic to K for which an obstruction of bounded branch size exists. The precise combinatorial and geometric structure of corresponding obstructions can be used to get a linear time algorithm for either finding an embedding extension or discovering minimal obstructions.