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Search: id:A098057
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| A098057 |
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Number of peakless Motzkin paths with no U H^j U, no D H^j D and no D H^jU (j>0), where U=(1,1), D=(1,-1) and H=(1,0) (can be easily expressed using RNA secondary structure terminology). |
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+0
2
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| 1, 1, 1, 2, 4, 8, 15, 27, 48, 84, 147, 257, 451, 796, 1413, 2526, 4544, 8226, 14978, 27417, 50434, 93183, 172865, 321857, 601263, 1126644, 2116968, 3987960, 7530200, 14249649, 27019301, 51327965, 97676156, 186177568, 355406479, 679425009
(list; graph; listen)
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OFFSET
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0,4
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REFERENCES
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I. L. Hofacker, P. Schuster and P. F. Stadler, Combinatorics of RNA secondary structures, Discrete Appl. Math., 88, 1998, 207-237.
P. R. Stein and M. S. Waterman, On some new sequences generalizing the Catalan and Motzkin numbers, Discrete Math., 26, 1979, 261-272.
M. Vauchassade de Chaumont and G. Viennot, Polynomes orthogonaux et problemes d'enumeration en biologie moleculaire, Publ. I.R.M.A. Strasbourg, 1984, 229/S-08, Actes 8e Sem. Lotharingien, pp. 79-86.
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LINKS
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M. Vauchassade de Chaumont and G. Viennot, Polynomes orthogonaux at problemes d'enumeration en biologie moleculaire, Sem. Loth. Comb. B08l (1984) 79-86.
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FORMULA
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G.f.=[1-z+z^2-4z^3+2z^4-sqrt(1-2z-z^2+2z^3+z^4-4z^5+4z^6)]/[2z^2*(1-z)^3].
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EXAMPLE
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a(4)=4 because we have HHHH, UHDU, HUHD and UHHD; a(6)=15 because from all 17 peakless Motzkin paths of length 6 (see A004148) only (UHU)HDD and UUH(DHD) do not qualify.
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CROSSREFS
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Cf. A004148.
Sequence in context: A001523 A000126 A143281 this_sequence A074029 A138653 A054159
Adjacent sequences: A098054 A098055 A098056 this_sequence A098058 A098059 A098060
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KEYWORD
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nonn
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AUTHOR
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Emeric Deutsch (deutsch(AT)duke.poly.edu), Sep 11 2004
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