We study the DT0L sequence equivalence problem for marked morphisms. We show that to decide this problem it is enough to consider initial terms involving at most 2n morphisms where n is the cardinality of the underlying alphabet.
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We show that it is decidable whether or not a given D0L power series and a given DF0L power series over a computable field are equal. This generalizes to power series the result of Ruohonen stating that DF0L-D0L language equivalence is decidable.
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We study D0L sequences and their equality sets. If s = (s(n))n≥0 and t = (t(n))n≥0 are D0L sequences, their equality set is defined by E(s, t) = {n ≥ 0 | s(n) = t(n)}. It is an open problem whether such equality sets are always eventually periodic. Using methods developed by Ehrenfeucht and Rozenberg we show that a D0L equality set is eventually periodic if it contains at least one infinite arithmetic progression. As a main tool we use elementary morphisms introduced by Ehrenfeucht and Rozenberg.
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We show that it is undecidable whether or not a given N-rational series assumes all nonnegative values. As a consequence we see that it is undecidable whether the image of a given N-rational series equals the image of a given N-rational sequence. We discuss also related results concerning DT0L and D0L length sets.
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We introduce a new class of algebraic series with noncommuting variables having a decidable equivalence problem. As a tool we consider star roots of formal power series.
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