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Description
We consider the minimum k-way cut problem for unweighted
undirected graphs with a size bound s on the number of cut edges
allowed. Thus we seek to remove as few edges as possible so as to
split a graph into k components, or report that this requires
cutting more than s edges. We show that this problem is
fixed-parameter tractable (FPT) with the standard parameterization
in terms of the solution size s. More precisely,
for s=O(1), we present a quadratic time algorithm. Moreover, we
present a much easier linear time algorithm for planar graphs and
bounded genus graphs.
Our tractability result stands in contrast to known W[1] hardness of
related problems. Without the size bound, Downey et al. [2003]
proved that the minimum k-way cut problem is W[1] hard
with parameter k, and this is even for simple unweighted graphs. Downey et
al. asked about the status for planar graphs. We get
linear time with
fixed parameter k for simple planar graphs since the minimum
k-way cut of a planar graph is of size at most 6k. More
generally, we get FTP with parameter k for any graph class with
bounded average degree.
A simple reduction shows that vertex cuts are at least as hard as
edge cuts, so the minimum k-way vertex cut is also W[1] hard with parameter
k. Marx [2004] proved that finding a minimum k-way
vertex cut of size s is also W[1] hard with parameter s. Marx asked about
the FPT status with edge cuts, which we prove tractable here. We are
not aware of any other cut problem where the vertex version is W[1] hard but the edge version is FPT, e.g., Marx [2004] proved
that the k-terminal cut problem is FTP parameterized by cut size,
both for edge and vertex cuts.