Pavel Klavík, A. Cristiano I. Malossi, et al.
Philos. Trans. R. Soc. A
Lengauer and Tarjan proved that the number of black and white pebbles needed to pebble the root of a tree is at least half the number of black pebbles needed to pebble the root. This result is extended to a larger class of acyclic directed graphs including pyramid graphs. © 1985, ACM. All rights reserved.