论文标题
多项式时间算法,用于到达类似树的多编码
Polynomial Time Algorithm for ARRIVAL on Tree-like Multigraphs
论文作者
论文摘要
在有向图中的转子行走可以被认为是马尔可夫链的确定性版本,在该链中,卵石按照简单的规则从顶点移动到顶点,直到达到终端顶点或汇。 Dohrau和Al。定义的到达问题包括确定将达到哪个下水道。虽然步行本身可以采取指数级的步骤,但此问题属于NP $ \ cap $ co-np的复杂性,而却不知道在P中。已经研究了几个变体,我们将一个或两个播放器添加到该模型中,定义了随机模型的确定性类似物(例如,Markovian决策过程,STOCHASCASIC GAMES,STOCHASTACTASING GEAMES,STOCHASTACTACT,STOCHASTACTACT GEAMES,STOCHASCASIC GAMES,STOCHASCASIC GAMESS)与Rotor-Ruftrutsrud ofers inters inters inters inters inters inters inters inters intersions。相应的决策问题解决了玩家的策略的存在,这些策略确保了到达的水槽上的某些条件。对于一个玩家来说,这些问题是$ NP $ -Complete,对于两个玩家来说是$ PSPACE $ -COMPLETE。在这项工作中,我们定义了一类有向图,即类似树状的多编码,它们是具有无向树的全局形状的多编码。我们证明,可及性问题的不同变体可以在线性时间内解决零,一个或两个播放器,而转子步行步骤的数量仍然可以指数。为了实现这一目标,我们定义了返回流的概念,该概念计算了卵石将在图形子树中反弹的次数。
A rotor walk in a directed graph can be thought of as a deterministic version of a Markov Chain, where a pebble moves from vertex to vertex following a simple rule until a terminal vertex, or sink, is reached. The ARRIVAL problem, as defined by Dohrau and al., consists in determining which sink will be reached. While the walk itself can take an exponential number of steps, this problem belongs to the complexity class NP$\cap$co-NP without being known to be in P. Several variants have been studied where we add one or two players to the model, defining deterministic analogs of stochastic models (e.g., Markovian decision processes, Stochastic Games) with rotor-routing rules instead of random transitions. The corresponding decision problem address the existence of strategies for players that ensure some condition on the reached sink. These problems are known to be $NP$-complete for one player and $PSPACE$-complete for two players. In this work, we define a class of directed graphs, namely tree-like multigraphs, which are multigraphs having the global shape of an undirected tree. We prove that the different variants of the reachability problem with zero, one, or two players can be solved in linear time, while the number of steps of rotor walks can still be exponential. To achieve this, we define a notion of return flow, which counts the number of times the pebble will bounce back in subtrees of the graph.