Creative Commons Attribution 3.0 Unported license
It is shown that every tree of size n over a fixed set of sigma different ranked symbols can be decomposed into O(n/log_sigma(n)) = O((n * log(sigma))/ log(n)) many hierarchically defined pieces. Formally, such a hierarchical decomposition has the form of a straight-line linear context-free tree grammar of size O(n/log_sigma(n)), which can be used as a compressed representation of the input tree. This generalizes an analogous result for strings. Previous grammar-based tree compressors were not analyzed for the worst-case size of the computed grammar, except for the top dag of Bille et al., for which only the weaker upper bound of O(n/log^{0.19}(n)) for unranked and unlabelled trees has been derived. The main result is used to show that every arithmetical formula of size n, in which only m <= n different variables occur, can be transformed (in time O(n * log(n)) into an arithmetical circuit of size O((n * log(m))/log(n)) and depth O(log(n)). This refines a classical result of Brent, according to which an arithmetical formula of size n can be transformed into a logarithmic depth circuit of size O(n).
@InProceedings{hucke_et_al:LIPIcs.FSTTCS.2014.457,
author = {Hucke, Danny and Lohrey, Markus and Noeth, Eric},
title = {{Constructing Small Tree Grammars and Small Circuits for Formulas}},
booktitle = {34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014)},
pages = {457--468},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-939897-77-4},
ISSN = {1868-8969},
year = {2014},
volume = {29},
editor = {Raman, Venkatesh and Suresh, S. P.},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2014.457},
URN = {urn:nbn:de:0030-drops-48639},
doi = {10.4230/LIPIcs.FSTTCS.2014.457},
annote = {Keywords: grammar-based compression, tree compression, arithmetical circuits}
}