We show that the st-connectivity problem for directed planar graphs can be decided in unambiguous logarithmic space.
Citation:
Chris Bourke, Raghunath Tewari, N.V. Vinodchandran, "Directed Planar Reachability is in Unambiguous Log-Space," ccc, pp.217-221, Twenty-Second Annual IEEE Conference on Computational Complexity (CCC'07), 2007