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A151640 Number of permutations of 4 indistinguishable copies of 1..n with exactly 2 adjacent element pairs in decreasing order. 2
0, 36, 1828, 40136, 693960, 11000300, 168594156, 2550000528, 38371094416, 576250000820, 8647558594740, 129734375001176, 1946130371095128, 29192578125001596, 437892028808595580, 6568398437500002080, 98526072692871096096, 1477891601562500002628 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

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

Index entries for linear recurrences with constant coefficients, signature (28,-253,976,-1675,1300,-375).

FORMULA

a(n) = 15^n - (4*n + 1)*5^n + 2*n*(4*n + 1). - Andrew Howroyd, May 06 2020

From Colin Barker, May 06 2020: (Start)

G.f.: 4*x^2*(9 + 205*x - 485*x^2 - 625*x^3) / ((1 - x)^3*(1 - 5*x)^2*(1 - 15*x)).

a(n) = 28*a(n-1) - 253*a(n-2) + 976*a(n-3) - 1675*a(n-4) + 1300*a(n-5) - 375*a(n-6) for n>6.

(End)

PROG

(PARI) a(n) = {15^n - (4*n + 1)*5^n + 2*n*(4*n + 1)} \\ Andrew Howroyd, May 06 2020

(PARI) concat(0, Vec(4*x^2*(9 + 205*x - 485*x^2 - 625*x^3) / ((1 - x)^3*(1 - 5*x)^2*(1 - 15*x)) + O(x^20))) \\ Colin Barker, May 07 2020

CROSSREFS

Column k=2 of A236463.

Sequence in context: A219986 A113618 A054980 * A025754 A071128 A065782

Adjacent sequences:  A151637 A151638 A151639 * A151641 A151642 A151643

KEYWORD

nonn,easy

AUTHOR

R. H. Hardin, May 29 2009

EXTENSIONS

Terms a(9) and beyond from Andrew Howroyd, May 06 2020

STATUS

approved

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Last modified August 10 12:36 EDT 2022. Contains 356039 sequences. (Running on oeis4.)