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A152499 1/12 of the number of permutations of 3 indistinguishable copies of 1..n with exactly 2 local maxima. 6
0, 1, 30, 664, 13632, 274432, 5497344, 109987840, 2199945216, 43999756288, 879998926848, 17599995314176, 351999979683840, 7039999912443904, 140799999624609792, 2815999998397775872, 56319999993188450304, 1126399999971143188480, 22527999999878130302976 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

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

Index entries for linear recurrences with constant coefficients, signature (28,-176,320).

FORMULA

a(n) = (11*20^(n-1) - 11*4^(n-1) - 12*(n-1)*4^(n-1))/128. - Andrew Howroyd, May 10 2020

From Colin Barker, Jul 15 2020: (Start)

G.f.: x^2*(1 + 2*x) / ((1 - 4*x)^2*(1 - 20*x)).

a(n) = 28*a(n-1) - 176*a(n-2) + 320*a(n-3) for n>2.

(End)

PROG

(PARI) a(n) = {(11*20^(n-1) - 11*4^(n-1) - 12*(n-1)*4^(n-1))/128} \\ Andrew Howroyd, May 10 2020

(PARI) concat(0, Vec(x^2*(1 + 2*x) / ((1 - 4*x)^2*(1 - 20*x)) + O(x^40))) \\ Colin Barker, Jul 15 2020

CROSSREFS

Cf. A152494, A334773.

Sequence in context: A111779 A075473 A051563 * A027475 A180801 A035520

Adjacent sequences:  A152496 A152497 A152498 * A152500 A152501 A152502

KEYWORD

nonn,easy

AUTHOR

R. H. Hardin, Dec 06 2008

EXTENSIONS

Terms a(10) and beyond from Andrew Howroyd, May 10 2020

STATUS

approved

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Last modified November 28 04:05 EST 2021. Contains 349400 sequences. (Running on oeis4.)