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Continued Fractions and Generalized Patterns

✍ Scribed by Toufik Mansour


Publisher
Elsevier Science
Year
2002
Tongue
English
Weight
140 KB
Volume
23
Category
Article
ISSN
0195-6698

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✦ Synopsis


Babson and Steingrimsson (2000, SΓ©minaire Lotharingien de Combinatoire, B44b, 18)

introduced generalized permutation patterns that allow the requirement that two adjacent letters in a pattern must be adjacent in the permutation.

Let f Ο„ ;r (n) be the number of 1-3-2-avoiding permutations on n letters that contain exactly r occurrences of Ο„ , where Ο„ is a generalized pattern on k letters. Let F Ο„ ;r (x) and F Ο„ (x, y) be the generating functions defined by F Ο„ ;r (x) = nβ‰₯0 f Ο„ ;r (n)x n and F Ο„ (x, y) = r β‰₯0 F Ο„ ;r (x)y r . We find an explicit expression for F Ο„ (x, y) in the form of a continued fraction for Ο„ given as a generalized pattern:

In particular, we find F Ο„ (x, y) for any Ο„ generalized pattern of length 3. This allows us to express F Ο„ ;r (x) via Chebyshev polynomials of the second kind and continued fractions.


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