The ambiguity of a nondeterministic finite automaton (NFA) $N$ for input size $n$ is the maximal number of accepting computations of $N$ for an input of size $n$. For all $k,r \in \mathbb{N}$ we construct languages $L_{r,k}$ which can be recognized by NFA's with size $k \cdot$poly$(r)$ and ambiguity $O(n^k)$, but $L_{r,k}$ has only NFA's with exponential size, if ambiguity $o(n^k)$ is required. In particular, a hierarchy for polynomial ambiguity is obtained, solving a long standing open problem (Ravikumar and Ibarra, 1989, Leung, 1998).
@InProceedings{hromkovic_et_al:LIPIcs.STACS.2009.1805, author = {Hromkovic, Juraj and Schnitger, Georg}, title = {{Ambiguity and Communication}}, booktitle = {26th International Symposium on Theoretical Aspects of Computer Science}, pages = {553--564}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-09-5}, ISSN = {1868-8969}, year = {2009}, volume = {3}, editor = {Albers, Susanne and Marion, Jean-Yves}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2009.1805}, URN = {urn:nbn:de:0030-drops-18054}, doi = {10.4230/LIPIcs.STACS.2009.1805}, annote = {Keywords: Nondeterministic finite automata, Ambiguity, Communication complexity} }
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