Tony Huynh, Eun Jung Kim, Sang-il Oum, Roohani Sharma, Marek Sokołowski
原文摘要
Let $E$ be a set of $n$ elements, $f\colon2^E\to\mathbb Z_{\ge0}$ be submodular, $t_1,\ldots,t_r\in E$ be distinct, and $k\in\mathbb N$. The Submodular Multiway Partition problem asks whether $E$ has a partition $(P_1,\ldots,P_r)$ with $t_i\in P_i$ for all $i$ and $\sum_i f(P_i)\le k$. We prove that this problem can be solved in $r^{O(k)}n^{O(1)}$ time whenever $f$ is posimodular. This covers monotone submodular functions, matroid rank functions, and nonnegative combinations of monotone and symmetric submodular functions. When $f$ is a connectivity function, we remove the dependence on the number of terminals and obtain a $k^{O(k)}n^{O(1)}$-time algorithm. The proof is entirely elementary, and is arguably the simplest proof that Edge Multiway Cut is fixed-parameter tractable. As a new consequence, our main result immediately implies that Hypergraph Multiway Partition is fixed-parameter tractable.
Rutger Campbell, Bruno Guillon, Mamadou Moustapha Kanté, Eun Jung Kim, Noleen Köhler
原文摘要
We give $\operatorname{CMSO}$-transductions that, given a graph $G$, output its modular decomposition, its split decomposition and its bi-join decomposition. This improves results by Courcelle [Logical Methods in Computer Science, 2006] who gave such transductions using order-invariant $\operatorname{MSO}$, a strictly more expressive logic than $\operatorname{CMSO}$. Our methods more generally yield $\operatorname{C}_2 \operatorname{MSO}$-transductions that output the canonical decompositions of weakly-partitive set systems and weakly-bipartitive systems of bipartitions.