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A000487 Number of permutations of length n with exactly two valleys.
(Formerly M5022 N2165)
+0
4
16, 272, 2880, 24576, 185856, 1304832, 8728576, 56520704, 357888000, 2230947840, 13754155008, 84134068224, 511780323328, 3100738912256, 18733264797696, 112949304754176, 680032201605120, 4090088616099840 (list; graph; listen)
OFFSET

5,1

REFERENCES

F. N. David, M. G. Kendall and D. E. Barton, Symmetric Function and Allied Tables, Cambridge, 1966, p. 261.

LINKS

R. G. Rieper and M. Zeleke, Valleyless Sequences

FORMULA

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

CROSSREFS

Cf. A000431, A000517.

Adjacent sequences: A000484 A000485 A000486 this_sequence A000488 A000489 A000490

Sequence in context: A068770 A113359 A119290 this_sequence A002303 A004382 A038758

KEYWORD

nonn

AUTHOR

njas

EXTENSIONS

More terms from Ralf Stephan (ralf(AT)ark.in-berlin.de), Sep 18 2003

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Last modified October 6 16:13 EDT 2008. Contains 144667 sequences.


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