TR10-009 Authors: A. Pavan, Raghunath Tewari, Vinodchandran Variyam

Publication: 15th January 2010 18:39

Downloads: 1733

Keywords:

We report progress on the \NL\ vs \UL\ problem.

\begin{itemize}

\item[-] We show unconditionally that the complexity class $\ReachFewL\subseteq\UL$. This improves on the earlier known upper bound $\ReachFewL \subseteq \FewL$.

\item[-] We investigate the complexity of min-uniqueness - a central

notion in studying the \NL\ vs \UL\ problem.

\begin{itemize}

\item We show that min-uniqueness is necessary and sufficient for showing

$\NL\ =\UL$.

\item We revisit the class $\OptL[\log n]$ and show that {\sc ShortestPathLength} - computing the length of the shortest path in a DAG, is complete for $\OptL[\log n]$.

\item We introduce $\UOptL[\log n]$, an unambiguous version of $\OptL[\log n]$, and show that (a) $\NL =\UL$ if and only if $\OptL[\log n] = \UOptL[\log n]$, (b) $\LogFew \leq \UOptL[\log n] \leq \SPL$.

\end{itemize}

\item[-] We show that the reachability problem over graphs embedded on 3 pages is complete for \NL. This contrasts with the reachability problem over graphs embedded on 2 pages which is logspace equivalent to the reachability problem in planar graphs and hence is in \UL.

\end{itemize}