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 A151583 Number of permutations of 2 indistinguishable copies of 1..n arranged in a circle with exactly 2 adjacent element pairs in decreasing order. 6
 0, 2, 45, 260, 1115, 4230, 15113, 52232, 176823, 590090, 1948133, 6376716, 20725523, 66960782, 215232705, 688746512, 2195381615, 6973567506, 22082966429, 69735686420, 219667415499, 690383309462, 2165293110905, 6778308873240, 21182215233575, 66088511533850 (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 (9,-30,46,-33,9). FORMULA a(n) = n*(3^n - 4*n) for n > 1. - Andrew Howroyd, May 04 2020 From Colin Barker, Jul 15 2020: (Start) G.f.: x^2*(2 + 27*x - 85*x^2 + 33*x^3 - 9*x^4) / ((1 - x)^3*(1 - 3*x)^2). a(n) = 9*a(n-1) - 30*a(n-2) + 46*a(n-3) - 33*a(n-4) + 9*a(n-5) for n>6. (End) PROG (PARI) a(n) = if(n <= 1, 0, n*(3^n - 4*n)) \\ Andrew Howroyd, May 04 2020 (PARI) concat(0, Vec(x^2*(2 + 27*x - 85*x^2 + 33*x^3 - 9*x^4) / ((1 - x)^3*(1 - 3*x)^2) + O(x^30))) \\ Colin Barker, Jul 15 2020 CROSSREFS With 3..8 descents: A151584, A151585, A151586, A151587, A151588, A151589. With 3..7 copies of 1..n: A151590, A151597, A151603, A151607, A151610. Cf. A151576, A151624. Sequence in context: A317432 A302468 A303250 * A325935 A360065 A222997 Adjacent sequences: A151580 A151581 A151582 * A151584 A151585 A151586 KEYWORD nonn,easy AUTHOR R. H. Hardin, May 21 2009 EXTENSIONS Terms a(13) and beyond from Andrew Howroyd, May 04 2020 STATUS approved

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Last modified September 19 14:26 EDT 2024. Contains 376012 sequences. (Running on oeis4.)