Constant Time Generation of Free Trees
Robert Wright, Bruce M. Richmond, Andrew M. Odlyzko, Brendan D. McKay · SIAM Journal on Computing · 1986
An algorithm of Beyer and Hedetniemi [SIAM J. Comput., 9 (1980), pp. 706–712] for generating rooted unlabeled trees is extended to generate unlabeled free trees. All the nonisomorphic trees of a given size are generated, without repetition, in time proportional to the number of trees.