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Search: id:A000487
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| A000487 |
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Number of permutations of length n with exactly two valleys.
(Formerly M5022 N2165)
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+0
4
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| 16, 272, 2880, 24576, 185856, 1304832, 8728576, 56520704, 357888000, 2230947840, 13754155008, 84134068224, 511780323328, 3100738912256, 18733264797696, 112949304754176, 680032201605120, 4090088616099840
(list; graph; listen)
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OFFSET
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5,1
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
F. N. David, M. G. Kendall and D. E. Barton, Symmetric Function and Allied Tables, Cambridge, 1966, p. 261.
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LINKS
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R. G. Rieper and M. Zeleke, Valleyless Sequences
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FORMULA
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G.f.: 16x^5(1-3x)/((1-2x)^3*(1-4x)^2*(1-6x)). - Ralf Stephan (ralf(AT)ark.in-berlin.de), Sep 18 2003
a(n) = (6^n + (2 - 2n)4^n + (2n^2 - 4n - 1)2^n)/32. - Mitchell Harris, Apr 02 2004
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CROSSREFS
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Cf. A000431, A000517.
Sequence in context: A161595 A144660 A158574 this_sequence A002303 A158610 A004382
Adjacent sequences: A000484 A000485 A000486 this_sequence A000488 A000489 A000490
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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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More terms from Ralf Stephan (ralf(AT)ark.in-berlin.de), Sep 18 2003
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