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A151624 Number of permutations of 2 indistinguishable copies of 1..n with exactly 2 adjacent element pairs in decreasing order. 5
0, 1, 48, 603, 5158, 37257, 247236, 1568215, 9703890, 59226357, 358722928, 2163496611, 13017647646, 78225458401, 469740168924, 2819689366191, 16922139539626, 101545622110989, 609314411814024, 3656015481903355, 21936500845191030, 131620291694585721 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..500

Index entries for linear recurrences with constant coefficients, signature (15,-84,226,-309,207,-54).

FORMULA

a(n) = 6^n - (2*n + 1)*3^n + n*(2*n + 1). - Andrew Howroyd, May 06 2020

From Colin Barker, Jul 16 2020: (Start)

G.f.: x^2*(1 + 33*x - 33*x^2 - 81*x^3) / ((1 - x)^3*(1 - 3*x)^2*(1 - 6*x)).

a(n) = 15*a(n-1) - 84*a(n-2) + 226*a(n-3) - 309*a(n-4) + 207*a(n-5) - 54*a(n-6) for n>6.

(End)

PROG

(PARI) a(n) = {6^n - (2*n + 1)*3^n + n*(2*n + 1)} \\ Andrew Howroyd, May 06 2020

(PARI) Vec(x^2*(1 + 33*x - 33*x^2 - 81*x^3) / ((1 - x)^3*(1 - 3*x)^2*(1 - 6*x)) + O(x^25)) \\ Colin Barker, Jul 16 2020

CROSSREFS

Column k=2 of A154283.

Sequence in context: A190601 A179404 A171343 * A187611 A160286 A008658

Adjacent sequences:  A151621 A151622 A151623 * A151625 A151626 A151627

KEYWORD

nonn,easy

AUTHOR

R. H. Hardin, May 29 2009

EXTENSIONS

Terms a(12) and beyond from Andrew Howroyd, May 06 2020

STATUS

approved

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Last modified April 23 05:30 EDT 2021. Contains 343199 sequences. (Running on oeis4.)