A151607 Number of permutations of 6 indistinguishable copies of 1..n arranged in a circle with exactly 2 adjacent element pairs in decreasing order.

0, 150, 2763, 28236, 251205, 2116386, 17292639, 138352920, 1089544473, 8474253870, 65251778163, 498286334052, 3778671399789, 28485369052602, 213640267939335, 1595180667331632, 11864156213337153, 87934334287152582, 649737025566256155, 4787535977856705660

Offset

1,2

Links

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

Index entries for linear recurrences with constant coefficients, signature (17,-94,190,-161,49).

Formula

a(n) = n*(3*7^n - 36*n) for n > 1. - Andrew Howroyd, May 04 2020

From Colin Barker, Jul 17 2020: (Start)

G.f.: 3*x^2*(50 + 71*x - 1545*x^2 + 805*x^3 - 245*x^4) / ((1 - x)^3*(1 - 7*x)^2).

a(n) = 17*a(n-1) - 94*a(n-2) + 190*a(n-3) - 161*a(n-4) + 49*a(n-5) for n>6.

(End)

Prog

(PARI) a(n) = if(n <= 1, 0, n*(3*7^n - 36*n)) \\ Andrew Howroyd, May 04 2020
(PARI) concat(0, Vec(3*x^2*(50 + 71*x - 1545*x^2 + 805*x^3 - 245*x^4) / ((1 - x)^3*(1 - 7*x)^2) + O(x^40))) \\ Colin Barker, Jul 17 2020

Crossrefs

Cf. A151583.

Sequence in context: A184665 A088361 A206041 * A135968 A135969 A128963

Adjacent sequences: A151604 A151605 A151606 * A151608 A151609 A151610

Keyword

nonn,easy

Author

R. H. Hardin, May 21 2009

Extensions

Terms a(7) and beyond from Andrew Howroyd, May 04 2020

Status

approved