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doi:10.1016/0304-3975(95)00168-9 
Copyright © 1996 Published by Elsevier Science B.V.
Regular paper
On almost cylindrical languages and the decidability of the D0L and PWD0L primitivity problems
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John Harrison
Communicated by G. Rozenberg
Abstract
Primitive words and their properties have always been of fundamental importance in the study of formal language theory. Head and Lando in Periodic D0L Languages proposed the idea of deciding whether or not a given D0L language has the property that every word in it is a primitive word. After reducing the general problem to the case in which h is injective, it will be shown that primitivity is decidable when ((A)h)* is an almost cylindrical set. Moreover, in this case, it is shown that the set of words which generate primitive sequences (given a particular D0L scheme) is an algorithmically constructible context-sensitive language. An undecidability result for the PWD0L primitivity problem and decidability results for cases of the RWD0L primitivity problem are also given.
