Practical Approximation Algorithms for Zero- and Bounded-Skew Trees.

Alexander Zelikovsky Ion I. Mandoiu Practical Approximation Algorithms for Zero- and Bounded-Skew Trees. 97-111 2001 15 SIAM J. Discrete Math. 1 http://epubs.siam.org/sam-bin/dbq/article/37836 db/journals/siamdm/siamdm15.html#ZelikovskyM01