Michael A. Bender Antonio Fernández Dana Ron Amit Sahai Salil P. Vadhan The Power of a Pebble: Exploring and Mapping Directed Graphs. 1-21 2002 176 Inf. Comput. 1 http://dx.doi.org/10.1006/inco.2001.3081 db/journals/iandc/iandc176.html#BenderFRSV02