A152494 1/3 of the number of permutations of 2 indistinguishable copies of 1..n with exactly 2 local maxima.

0, 1, 19, 235, 2539, 26119, 263863, 2648107, 26513875, 265250287, 2652876847, 26530008499, 265304159371, 2653054879735, 26530591844071, 265306057146811, 2653061016284227, 26530611583384063, 265306120353746335, 2653061217872021443, 26530612224048411643

Offset

1,3

Links

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

Index entries for linear recurrences with constant coefficients, signature (16,-69,90).

Formula

a(n) = (13*10^(n-1) - 13*3^(n-1) - 14*(n-1)*3^(n-1))/49. - Andrew Howroyd, May 10 2020

From Colin Barker, May 19 2020: (Start)

G.f.: x*(1 + 3*x) / ((1 - 3*x)^2*(1 - 10*x)).

a(n) = 16*a(n-1) - 69*a(n-2) + 90*a(n-3) for n>3.

(End)

Prog

(PARI) a(n) = {(13*10^(n-1) - 13*3^(n-1) - 14*(n-1)*3^(n-1))/49} \\ Andrew Howroyd, May 10 2020
(PARI) concat(0, Vec(x*(1 + 3*x) / ((1 - 3*x)^2*(1 - 10*x)) + O(x^20))) \\ Colin Barker, May 19 2020

Crossrefs

Cf. A334773.

Sequence in context: A025965 A276201 A025960 * A171158 A022033 A025938

Adjacent sequences: A152491 A152492 A152493 * A152495 A152496 A152497

Keyword

nonn,easy

Author

R. H. Hardin, Dec 06 2008

Extensions

Terms a(12) and beyond from Andrew Howroyd, May 10 2020

Status

approved