Philippe Flajolet, Helmut Prodinger. Level number sequences for trees. Discrete Mathematics, 65(2):149-156, 1987. [doi]
@article{FlajoletP87,
title = {Level number sequences for trees},
author = {Philippe Flajolet and Helmut Prodinger},
year = {1987},
doi = {10.1016/0012-365X(87)90137-3},
url = {http://dx.doi.org/10.1016/0012-365X(87)90137-3},
researchr = {https://researchr.org/publication/FlajoletP87},
cites = {0},
citedby = {0},
journal = {Discrete Mathematics},
volume = {65},
number = {2},
pages = {149-156},
}