Abstract
An infinite word has the property R m if every factor has exactly m return words. Vuillon showed that R 2 characterizes Sturmian words. We prove that a word satisfies R m if its complexity function is (m − 1)n + 1 and if it contains no weak bispecial factor. These conditions are necessary for m = 3, whereas for m = 4 the complexity function need not be 3n + 1. A new class of words satisfying R m is given.
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Balková, L., Pelantová, E. & Steiner, W. Sequences with constant number of return words. Monatsh Math 155, 251–263 (2008). https://doi.org/10.1007/s00605-008-0001-2
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DOI: https://doi.org/10.1007/s00605-008-0001-2