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arXiv 2403.04589cs.DScs.DMmath.CO
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时态有向无环图路径覆盖的算法与复杂度:Dilworth定理何时是动态的?

Algorithms and complexity for path covers of temporal DAGs: when is Dilworth dynamic?

Dibyayan Chakraborty, Antoine Dailly, Florent Foucaud, Ralf Klasing

AI总结:

本文研究时态DAG的最小时态路径覆盖(TPC)与时态不相交路径覆盖(TD-PC)的复杂度,证明多数场景下二者为NP难,同时给出部分特殊图类的多项式算法及以最大时间步加基础图树宽为参数的参数化算法。

摘要:

本文研究路径覆盖问题的动态对应版本,该问题在有向无环图中可在多项式时间内求解。时态有向图的弧集随离散时间步变化,若其基础有向图(所有弧集的并集)是无环的,则称为时态DAG。时态路径是基础有向图中的一条有向路径,满足沿路径的弧的时间步严格递增。若两条时态路径不会在同一时间占据任何顶点,则称它们是时态不相交的。时态(对应地,时态不相交)路径覆盖是覆盖所有顶点的(对应地,时态不相交的)时态路径的集合。本文研究寻找基数最小的时态(不相交)路径覆盖问题的计算复杂度,这两个问题分别记为Temporal Path Cover (TPC,时态路径覆盖)和Temporally Disjoint Path Cover (TD-PC,时态不相交路径覆盖)。我们证明,即使基础DAG是平面的、二分的、次立方的,且仅存在两个弧不相交的时间步,这两个问题也都是NP难的。此外,TD-PC即使在时态有向树上也保持NP难。与之相对,我们通过归约为(静态无向)弱弦图(完美图的一个子类,团覆盖问题在该类图上存在高效算法)的Clique Cover(团覆盖)问题,证明TPC在时态有向树上可在多项式时间内求解。这凸显了两个问题之间有趣的算法差异。尽管TD-PC在时态有向树上是NP难的,但它在时态有向线和时态有根有向树上可在多项式时间内求解。我们还证明,TPC(对应地,TD-PC)关于参数tmax + tw存在XP(对应地,FPT)时间算法,其中tmax是最大时间步,tw是基础静态无向图的树宽。

英文摘要:

In this paper, we study a dynamic analogue of the Path Cover problem, which can be solved in polynomial-time in directed acyclic graphs. A temporal digraph has an arc set that changes over discrete time-steps, if the underlying digraph (the union of all the arc sets) is acyclic, then we have a temporal DAG. A temporal path is a directed path in the underlying digraph, such that the time-steps of arcs are strictly increasing along the path. Two temporal paths are temporally disjoint if they do not occupy any vertex at the same time. A temporal (resp. temporally disjoint) path cover is a collection of (resp. temporally disjoint) temporal paths that covers all vertices. In this paper, we study the computational complexities of the problems of finding a temporal (disjoint) path cover with minimum cardinality, denoted as Temporal Path Cover (TPC) and Temporally Disjoint Path Cover (TD-PC). We show that both problems are NP-hard even when the underlying DAG is planar, bipartite, subcubic, and there are only two arc-disjoint time-steps. Moreover, TD-PC remains NP-hard even on temporal oriented trees. In contrast, we show that TPC is polynomial-time solvable on temporal oriented trees by a reduction to Clique Cover for (static undirected) weakly chordal graphs (a subclass of perfect graphs for which Clique Cover admits an efficient algorithm). This highlights an interesting algorithmic difference between the two problems. Although it is NP-hard on temporal oriented trees, TD-PC becomes polynomial-time solvable on temporal oriented lines and temporal rooted directed trees. We also show that TPC (resp. TD-PC) admits an XP (resp. FPT) time algorithm with respect to parameter tmax + tw, where tmax is the maximum time-step, and tw is the treewidth of the underlying static undirected graph.

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