A normal sequence over {0,1} is an infinite sequence for which every word of length k appears with frequency 2^{-k}. Agafonov’s eponymous theorem states that selection by a finite state selector preserves normality, i.e. if α is a normal sequence and A is a finite state selector, then the subsequence A(α) is either finite or a normal sequence. In this work, we address the following question: does this result hold when considering probabilistic selectors? We provide a partial positive answer, in the case where the probabilities involved are rational. More formally, we prove that given a normal sequence α and a rational probabilistic selector P, the selected subsequence P(α) will be a normal sequence with probability 1.
@InProceedings{lechine_et_al:LIPIcs.MFCS.2024.67, author = {L\'{e}chine, Ulysse and Seiller, Thomas and Simonsen, Jakob Grue}, title = {{Agafonov’s Theorem for Probabilistic Selectors}}, booktitle = {49th International Symposium on Mathematical Foundations of Computer Science (MFCS 2024)}, pages = {67:1--67:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-335-5}, ISSN = {1868-8969}, year = {2024}, volume = {306}, editor = {Kr\'{a}lovi\v{c}, Rastislav and Ku\v{c}era, Anton{\'\i}n}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2024.67}, URN = {urn:nbn:de:0030-drops-206238}, doi = {10.4230/LIPIcs.MFCS.2024.67}, annote = {Keywords: Normal sequences, probabilistic automata, Agafonov’s theorem} }
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