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A151625 Number of permutations of 2 indistinguishable copies of 1..n with exactly 3 adjacent element pairs in decreasing order. 3
0, 0, 20, 1168, 27664, 450048, 6030140, 72338144, 811888600, 8742609264, 91700484132, 945739871600, 9647920866016, 97729381122976, 985430937715404, 9906926101717184, 99407643757824680, 996242539625306512, 9976235831375328564, 99850100363120616144 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..200

FORMULA

a(n) = 10^n - (2*n + 1)*6^n + binomial(2*n+1, 2)*3^n - binomial(2*n+1, 3). - Andrew Howroyd, May 07 2020

PROG

(PARI) a(n) = {10^n - (2*n + 1)*6^n + binomial(2*n+1, 2)*3^n - binomial(2*n+1, 3)} \\ Andrew Howroyd, May 07 2020

CROSSREFS

Column k=3 of A154283.

Sequence in context: A091535 A265654 A152130 * A203775 A184269 A042745

Adjacent sequences:  A151622 A151623 A151624 * A151626 A151627 A151628

KEYWORD

nonn

AUTHOR

R. H. Hardin, May 29 2009

EXTENSIONS

Terms a(12) and beyond from Andrew Howroyd, May 07 2020

STATUS

approved

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Last modified April 22 14:16 EDT 2021. Contains 343177 sequences. (Running on oeis4.)