Abstract
In a breakthrough result, Reingold [17] showed that the Undirected st-Connectivity problem can be solved in O(log n) space. The next major challenge in this direction is whether one can extend it to directed graphs, and thereby lowering the deterministic space complexity of \(\mathcal{RL}\) or \(\mathcal{NL}\). In this paper, we show that Reingold’s algorithm, the O(log4/3 n)-space algorithm by Armoni et al.[3] and the O(log3/2 n)-space algorithm by Nisan et al.[14] can all be carried out on the RAM-NNJAG model [15](a uniform version of the NNJAG model [16]). As there is a tight Ω(log2 n) space lower bound for the Directed st-Connectivity problem on the RAM-NNJAG model implied by [8], our result gives an obstruction to generalizing Reingold’s algorithm to the directed case.
The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China [CityU 1071/02E], an NSF Grant, USA (CCR-0208013) and the Natural Science Foundation of China (60223004, 60321002).
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Lu, P., Zhang, J., Poon, C.K., Cai, JY. (2005). Simulating Undirected st-Connectivity Algorithms on Uniform JAGs and NNJAGs. In: Deng, X., Du, DZ. (eds) Algorithms and Computation. ISAAC 2005. Lecture Notes in Computer Science, vol 3827. Springer, Berlin, Heidelberg. https://doi.org/10.1007/11602613_77
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DOI: https://doi.org/10.1007/11602613_77
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