Critical exponent of a word

In mathematics and computer science, the critical exponent of a finite or infinite sequence of symbols over a finite alphabet describes the largest number of times a contiguous subsequence can be repeated. For example, the critical exponent of "Mississippi" is 7/3, as it contains the string "ississi", which is of length 7 and period 3.

If w is an infinite word over the alphabet A and x is a finite word over A, then x is said to occur in w with exponent α, for positive real α, if there is a factor y of w with y = xax0 where x0 is a prefix of x, a is the integer part of α, and the length |y| ≥ α |x|: we say that y is an α-power. The word w is α-power-free if it contains no factors which are α-powers.[1]

The critical exponent for w is the supremum of the α for which w has α-powers,[2] or equivalently the infimum of the α for which w is α-power-free.[3]

Examples
  • The critical exponent of the Fibonacci word is (5 + √5)/2 ≈ 3.618.[3][4]
  • The critical exponent of the Thue–Morse sequence is 2.[3] The word contains arbitrarily long squares, but in any factor xxb the letter b is not a prefix of x.

Properties
  • The critical exponent can take any real value greater than 1.[3][5]
  • The critical exponent of a morphic word over a finite alphabet is either infinite or an algebraic number of degree at most the size of the alphabet.[3]

Repetition threshold

The repetition threshold of an alphabet A of n letters is the minimum critical exponent of infinite words over A: clearly this value RT(n) depends only on n. For n=2, any binary word of length four has a factor of exponent 2, and since the critical exponent of the Thue–Morse sequence is 2, the repetition threshold for binary alphabets is RT(2) = 2. It is known that RT(3) = 7/4, RT(4) = 7/5 and that for n≥5 we have RT(n) ≥ n/(n-1). It is conjectured that the latter is the true value, and this has been established for 5 ≤ n ≤ 14 and for n ≥ 33.[2][4] Recently, M. Rao completed the proof for all values of n.

See also

Notes
  1. Krieger (2006) p.281
  2. Berstel et al (2009) p.126
  3. Krieger (2006) p.280
  4. Allouche & Shallit (2003) p. 37
  5. Krieger & Shallit (2007).

References
  • Allouche, Jean-Paul; Shallit, Jeffrey (2003). Automatic Sequences: Theory, Applications, Generalizations. Cambridge University Press. ISBN 978-0-521-82332-6. Zbl 1086.11015.
  • Berstel, Jean; Lauve, Aaron; Reutenauer, Christophe; Saliola, Franco V. (2009). Combinatorics on words. Christoffel words and repetitions in words. CRM Monograph Series. 27. Providence, RI: American Mathematical Society. ISBN 978-0-8218-4480-9. Zbl 1161.68043.
  • Krieger, Dalia (2006). "On critical exponents in fixed points of non-erasing morphisms". In Ibarra, Oscar H.; Dang, Zhe (eds.). Developments in Language Theory: Proceedings 10th International Conference, DLT 2006, Santa Barbara, CA, USA, June 26–29, 2006. Lecture Notes in Computer Science. 4036. Springer-Verlag. pp. 280–291. ISBN 3-540-35428-X. Zbl 1227.68074.
  • Krieger, D.; Shallit, J. (2007). "Every real number greater than one is a critical exponent". Theor. Comput. Sci. 381: 177–182. doi:10.1016/j.tcs.2007.04.037.
  • Lothaire, M. (2011). Algebraic combinatorics on words. Encyclopedia of Mathematics and Its Applications. 90. With preface by Jean Berstel and Dominique Perrin (Reprint of the 2002 hardback ed.). Cambridge University Press. ISBN 978-0-521-18071-9. Zbl 1221.68183.
  • Pytheas Fogg, N. (2002). Berthé, Valérie; Ferenczi, Sébastien; Mauduit, Christian; Siegel, A. (eds.). Substitutions in dynamics, arithmetics and combinatorics. Lecture Notes in Mathematics. 1794. Berlin: Springer-Verlag. ISBN 3-540-44141-7. Zbl 1014.11015.
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