一句话:证明带子词序的上下文无关字符串约束一般不可判定,无环依赖片段为 NEXPTime 完全,并确定无环有损通道下推系统控制状态可达性的复杂度。
本文研究一类字符串约束的可满足性:变量需满足上下文无关语言的成员约束,并满足变量之间的子词序关系。作者首先指出该变体整体上是不可判定的。随后,他们考虑一个受限片段,其中子词序约束不在变量之间引入循环依赖,并证明该片段的可满足性是 NEXPTime 完全的。作为结果的应用,作者确定了无环有损通道下推系统中控制状态可达性问题的复杂度:该问题此前已知可判定,但没有基本上界,本文证明其同样是 NEXPTime 完全的。
We consider a variant of string constraints given by membership constraints in context-free languages and subword relation between variables. The satisfiability problem for this variant turns out to be undecidable. We consider a fragment in which the subword-order constraints do not impose any cyclic dependency between variables. We show that this fragment is NexpTime-complete. As an application of our result, we settle the complexity of control state reachability in acyclic lossy channel pushdown systems, an important distributed system model. The problem was shown to be decidable in [8]. However, no elementary upper bound was known. We show that this problem is NexpTime-complete.