Computing the vertex separation of unicyclic graphs

We describe an <f>O(nlogn)</f> algorithm for the computation of the vertex separation of unicyclic graphs. The algorithm also computes a linear layout with optimal vertex separation in the same time bound. Pathwidth, node search number and vertex separation are different ways of defining the same notion. Path decompositions and search strategies can be derived from linear layouts. The algorithm applies existing, linear time, techniques for the computation of the vertex separation of trees together with corresponding optimal layouts. We reformulate the earlier work on the linear time computation of optimal layouts. A polynomial time algorithm for the problem on unicyclic graphs can be inferred from existing more general methods for graphs of fixed treewidth, since unicyclic graphs have treewidth two, but the time complexity of the resulting method would seem to be an inordinately high order polynomial. Our algorithm we claim is “practical.” The addition of one edge to a tree does seem to require a considerably more elaborate algorithm. [Copyright &y& Elsevier]