19 research outputs found
On the critical exponent of generalized Thue-Morse words
For certain generalized Thue-Morse words t, we compute the "critical
exponent", i.e., the supremum of the set of rational numbers that are exponents
of powers in t, and determine exactly the occurrences of powers realizing it.Comment: 13 pages; to appear in Discrete Mathematics and Theoretical Computer
Science (accepted October 15, 2007
The Critical Exponent is Computable for Automatic Sequences
The critical exponent of an infinite word is defined to be the supremum of
the exponent of each of its factors. For k-automatic sequences, we show that
this critical exponent is always either a rational number or infinite, and its
value is computable. Our results also apply to variants of the critical
exponent, such as the initial critical exponent of Berthe, Holton, and Zamboni
and the Diophantine exponent of Adamczewski and Bugeaud. Our work generalizes
or recovers previous results of Krieger and others, and is applicable to other
situations; e.g., the computation of the optimal recurrence constant for a
linearly recurrent k-automatic sequence.Comment: In Proceedings WORDS 2011, arXiv:1108.341
Bispecial factors in circular non-pushy D0L languages
We study bispecial factors in fixed points of morphisms. In particular, we
propose a simple method of how to find all bispecial words of non-pushy
circular D0L-systems. This method can be formulated as an algorithm. Moreover,
we prove that non-pushy circular D0L-systems are exactly those with finite
critical exponent.Comment: 18 pages, 5 figure
Periodicity, repetitions, and orbits of an automatic sequence
We revisit a technique of S. Lehr on automata and use it to prove old and new
results in a simple way. We give a very simple proof of the 1986 theorem of
Honkala that it is decidable whether a given k-automatic sequence is ultimately
periodic. We prove that it is decidable whether a given k-automatic sequence is
overlap-free (or squareefree, or cubefree, etc.) We prove that the
lexicographically least sequence in the orbit closure of a k-automatic sequence
is k-automatic, and use this last result to show that several related
quantities, such as the critical exponent, irrationality measure, and
recurrence quotient for Sturmian words with slope alpha, have automatic
continued fraction expansions if alpha does.Comment: preliminary versio