Given n subspaces of a finite-dimensional vector space over a fixed finite
field F, we wish to find a "branch-decomposition" of these subspaces
of width at most k, that is a subcubic tree T with n leaves mapped
bijectively to the subspaces such that for every edge e of T, the sum of
subspaces associated with leaves in one component of Tβe and the sum of
subspaces associated with leaves in the other component have the intersection
of dimension at most k. This problem includes the problems of computing
branch-width of F-represented matroids, rank-width of graphs,
branch-width of hypergraphs, and carving-width of graphs.
We present a fixed-parameter algorithm to construct such a
branch-decomposition of width at most k, if it exists, for input subspaces of
a finite-dimensional vector space over F. Our algorithm is analogous
to the algorithm of Bodlaender and Kloks (1996) on tree-width of graphs. To
extend their framework to branch-decompositions of vector spaces, we developed
highly generic tools for branch-decompositions on vector spaces. The only known
previous fixed-parameter algorithm for branch-width of F-represented
matroids was due to Hlin\v{e}n\'y and Oum (2008) that runs in time O(n3)
where n is the number of elements of the input F-represented
matroid. But their method is highly indirect. Their algorithm uses the
non-trivial fact by Geelen et al. (2003) that the number of forbidden minors is
finite and uses the algorithm of Hlin\v{e}n\'y (2005) on checking monadic
second-order formulas on F-represented matroids of small
branch-width. Our result does not depend on such a fact and is completely
self-contained, and yet matches their asymptotic running time for each fixed
k.Comment: 73 pages, 10 figure