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Search: id:A033305
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| A033305 |
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Number of permutations (p1,...,pn) such that 1<=|pk-k|<=2 for all k. |
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+0
1
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| 1, 0, 1, 2, 4, 6, 13, 24, 45, 84, 160, 300, 565, 1064, 2005, 3774, 7108, 13386, 25209, 47472, 89401, 168360, 317056, 597080, 1124425, 2117520, 3987721, 7509690, 14142276, 26632782, 50154949, 94451976, 177872293
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OFFSET
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0,4
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REFERENCES
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Lehmer, D. H.; Permutations with strongly restricted displacements. Combinatorial theory and its applications, II (Proc. Colloq., Balatonfured, 1969), pp. 755-770. North-Holland, Amsterdam, 1970.
R. P. Stanley, Enumerative Combinatorics I, p. 252, Example 4.7.16.
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FORMULA
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G.f.: (1-x)/(1-x-x^2-x^3-x^4+x^5).
a(n) = a(n-1) + a(n-2) + a(n-3) + a(n-4) - a(n-5)
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CROSSREFS
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Sequence in context: A058598 A087549 A109078 this_sequence A105543 A027712 A138307
Adjacent sequences: A033302 A033303 A033304 this_sequence A033306 A033307 A033308
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
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New description from Max Alekseyev (maxale(AT)gmail.com), Jul 09 2006
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