Consider a random permutation drawn from the set of permutations of length n that avoid a given set of one or several patterns of length 3. We show that the number of occurrences of another pattern has a limit distribution, after suitable scaling. In several cases, the limit is normal, as it is in the case of unrestricted random permutations; in other cases the limit is a non-normal distribution, depending on the studied pattern. In the case when a single pattern of length 3 is forbidden, the limit distributions can be expressed in terms of a Brownian excursion. The analysis is made case by case; unfortunately, no general method is known, and no general pattern emerges from the results.
@InProceedings{janson:LIPIcs.AofA.2018.6, author = {Janson, Svante}, title = {{Patterns in Random Permutations Avoiding Some Other Patterns}}, booktitle = {29th International Conference on Probabilistic, Combinatorial and Asymptotic Methods for the Analysis of Algorithms (AofA 2018)}, pages = {6:1--6:12}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-078-1}, ISSN = {1868-8969}, year = {2018}, volume = {110}, editor = {Fill, James Allen and Ward, Mark Daniel}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.AofA.2018.6}, URN = {urn:nbn:de:0030-drops-88996}, doi = {10.4230/LIPIcs.AofA.2018.6}, annote = {Keywords: Random permutations, patterns, forbidden patterns, limit in distribution, U-statistics} }
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