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arXiv 2104.07466cs.LOcs.GT
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概率验证的符号时间和空间权衡

Symbolic Time and Space Tradeoffs for Probabilistic Verification

Krishnendu Chatterjee, Wolfgang Dvořák, Monika Henzinger, Alexander Svozil

摘要:

我们提出了一个更快的符号算法的概率验证中的中心问题:计算马尔可夫决策过程(MDP)的最大端分量(MEC)分解。这个问题推广了图的SCC分解问题和马氏链的闭递归集问题。符号算法的模型广泛用于形式验证和模型检查,其中对输入模型的访问仅限于符号操作(例如,基本集合运算和一步邻域的计算)。对于具有$n$顶点和$m$边的输入MDP,20世纪90年代用于MEC分解的经典符号算法需要$O(n^2)$符号运算和$O(1)$符号空间。用于MEC分解的唯一其它符号算法需要$O(n \sqrt{m})$符号运算和$O(\sqrt{m})$符号空间。一个主要的未决问题是,符号运算的最坏情况$O(n^2)$是否可以被击败。我们提出了一个符号算法,需要$\widetilde{O}(n^{1.5})$符号操作和$\widetilde{O}(\sqrt{n})$符号空间。此外,我们的算法的参数化提供了符号操作和符号空间之间的权衡:对于所有的$0<ε\leq 1/2$的符号算法需要$\widetilde{O}(n^{2-ε})$符号操作和$\widetilde{O}(n^ε)$符号空间($\widetilde{O}$隐藏多对数因子)。使用我们的技术,我们提出了更快的算法,用于计算$ω$的几乎肯定获胜的地区-MDPs的常规目标。我们认为$ω$ -定期目标的规范奇偶校验目标,并与$d$ -优先级的奇偶校验目标,我们提出了一个算法,计算几乎肯定获胜的区域与$\widetilde{O}(n^{2-ε})$符号操作和$\widetilde{O}(n^ε)$符号空间,所有$0 < ε\leq 1/2$。

英文摘要:

We present a faster symbolic algorithm for the following central problem in probabilistic verification: Compute the maximal end-component (MEC) decomposition of Markov decision processes (MDPs). This problem generalizes the SCC decomposition problem of graphs and closed recurrent sets of Markov chains. The model of symbolic algorithms is widely used in formal verification and model-checking, where access to the input model is restricted to only symbolic operations (e.g., basic set operations and computation of one-step neighborhood). For an input MDP with $n$ vertices and $m$ edges, the classical symbolic algorithm from the 1990s for the MEC decomposition requires $O(n^2)$ symbolic operations and $O(1)$ symbolic space. The only other symbolic algorithm for the MEC decomposition requires $O(n \sqrt{m})$ symbolic operations and $O(\sqrt{m})$ symbolic space. A main open question is whether the worst-case $O(n^2)$ bound for symbolic operations can be beaten. We present a symbolic algorithm that requires $\widetilde{O}(n^{1.5})$ symbolic operations and $\widetilde{O}(\sqrt{n})$ symbolic space. Moreover, the parametrization of our algorithm provides a trade-off between symbolic operations and symbolic space: for all $0<ε\leq 1/2$ the symbolic algorithm requires $\widetilde{O}(n^{2-ε})$ symbolic operations and $\widetilde{O}(n^ε)$ symbolic space ($\widetilde{O}$ hides poly-logarithmic factors). Using our techniques we present faster algorithms for computing the almost-sure winning regions of $ω$-regular objectives for MDPs. We consider the canonical parity objectives for $ω$-regular objectives, and for parity objectives with $d$-priorities we present an algorithm that computes the almost-sure winning region with $\widetilde{O}(n^{2-ε})$ symbolic operations and $\widetilde{O}(n^ε)$ symbolic space, for all $0 < ε\leq 1/2$.

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