Treewidth Reduction for Constrained Separation and Bipartization Problems

We present a method for reducing the treewidth of a graph while preserving all the minimal s − t separators. This technique turns out to be very useful for establishing the fixedparameter tractability of constrained separation and bipartization problems. To demonstrate the power of this technique, we prove the fixed-parameter tractability of a number of well-known separation and bipartization problems with various additional restrictions (e.g., the vertices being removed from the graph form an independent set). These results answer a number of open questions in the area of parameterized complexity.