1. Reachability is harder for directed than for undirected graphs;Ajtai;J. Symbolic Logic,1990
2. Random walks, universal traversal sequences, and the complexity of the maze problem;Aleliunas,1979
3. Time-space tradeoffs for undirected graph traversal;Beame,1990
4. On the power of the compass;Blum,1978
5. On the capability of finite automata in 2 and 3 dimensional space;Blum,1977