arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1005.2642cs.CC
收藏

用于树评价的Pebbles和分枝程序

Pebbles and Branching Programs for Tree Evaluation

Stephen Cook, Pierre McKenzie, Dustin Wehr, Mark Braverman, Rahul Santhanam

摘要:

我们介绍了树评价问题,表明它是在logDCFL(因此在P),并研究其分支程序的复杂性,希望最终证明一个超对数空间下界。该问题的输入是一个高度为h的有根平衡d元树,其内部节点用d元函数标记在[k] = {1,.,k},并且其叶用[k]的元素标记。每个节点在[k]中获得一个值,该值等于其应用于其d个子节点的值的d元函数。输出是根的值。我们表明,标准的黑卵石算法应用于二叉树的高度h产生一个确定性的k路分支程序与Theta(k^h)状态解决这个问题,我们证明了这个上限是紧的h=2和h=3。我们介绍了一个简单的语义限制称为“节俭”的k路分支程序解决树的评价问题,并表明,相同的状态界θ(k^h)是紧(一个常数因子)的所有h >= 2的确定性节俭计划。我们介绍了分数卵石树,并表明,这产生不确定性节俭计划与Theta(k^{h/2+1})状态解决布尔问题“确定根是否有值1”。我们证明了这个界对于h= 2,3,4是紧的,并且对于h= 2,3的无限制非确定k路分支规划是紧的.

英文摘要:

We introduce the Tree Evaluation Problem, show that it is in logDCFL (and hence in P), and study its branching program complexity in the hope of eventually proving a superlogarithmic space lower bound. The input to the problem is a rooted, balanced d-ary tree of height h, whose internal nodes are labeled with d-ary functions on [k] = {1,...,k}, and whose leaves are labeled with elements of [k]. Each node obtains a value in [k] equal to its d-ary function applied to the values of its d children. The output is the value of the root. We show that the standard black pebbling algorithm applied to the binary tree of height h yields a deterministic k-way branching program with Theta(k^h) states solving this problem, and we prove that this upper bound is tight for h=2 and h=3. We introduce a simple semantic restriction called "thrifty" on k-way branching programs solving tree evaluation problems and show that the same state bound of Theta(k^h) is tight (up to a constant factor) for all h >= 2 for deterministic thrifty programs. We introduce fractional pebbling for trees and show that this yields nondeterministic thrifty programs with Theta(k^{h/2+1}) states solving the Boolean problem "determine whether the root has value 1". We prove that this bound is tight for h=2,3,4, and tight for unrestricted nondeterministic k-way branching programs for h=2,3.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

↑