The complexity of planarity testing

We clarify the computational complexity of planarity testing, by showing that planarity testing is hard for L, and lies in SL. This nearly settles the question, since it is widely conjectured that L = SL. The upper bound of SL matches the lower bound of L in the context of (non-uniform) circuit complexity, since L/poly is equal to SL/poly. Similarly, we show that a planar embedding, when one exists, can be found in FLSL. Previously, these problems were known to reside in the complexity class AC(1), via the O(log n) time CRCW-PRAM algorithm of Ramachandran and Reif, although planarity checking for degree-three graphs had been shown to be in SL. (C) 2003 Elsevier Inc. All rights reserved.