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A114938 Number of permutations of the multiset {1,1,2,2,...,n,n} with no two consecutive terms equal. 24
1, 0, 2, 30, 864, 39480, 2631600, 241133760, 29083420800, 4467125013120, 851371260364800, 197158144895712000, 54528028997584665600, 17752366094818747392000, 6720318485119046923315200, 2927066537906697348594432000, 1453437879238150456164433920000 (list; graph; refs; listen; history; text; internal format)



a(n) is also the number of (0,1)-matrices A=(a_ij) of size n X 2n such that each row has exactly two 1's and each column has exactly one 1 and with the restriction that no 1 stands on the line from a_11 to a_22. - Shanzhen Gao, Feb 24 2010

a(n) is the number of permutations of the multiset {1,1,2,2,...,n,n} with no fixed points. - Alexander Burstein, May 16 2020

Also the number of 2-uniform ordered set partitions of {1...2n} containing no two successive vertices in the same block. - Gus Wiseman, Jul 04 2020


R. P. Stanley, Enumerative Combinatorics Volume I, Cambridge University Press, 1997. Chapter 2, Sieve Methods, Example 2.2.3, page 68.


Seiichi Manyama, Table of n, a(n) for n = 0..238 (terms 1..100 from Andrew Woods)

H. Eriksson, A. Martin, Enumeration of Carlitz multipermutations, arXiv:1702.04177 [math.CO], 2017.


a(n) = Sum_{k=0..n} ((binomial(n, k)*(-1)^(n-k)*(n+k)!)/2^k).

a(n) = (-1)^n * n! * A000806(n), n>0. - Vladeta Jovovic, Nov 19 2009

a(n) = n*(2*n-1)*a(n-1) + (n-1)*n*a(n-2). - Vaclav Kotesovec, Aug 07 2013

a(n) ~ 2^(n+1)*n^(2*n)*sqrt(Pi*n)/exp(2*n+1). - Vaclav Kotesovec, Aug 07 2013

a(n) = n! * A278990(n). - Alexander Burstein, May 16 2020


a(2) = 2 because there are two permutations of {1,1,2,2} avoiding equal consecutive terms: 1212 and 2121.


Table[Sum[Binomial[n, i](2n-i)!/2^(n-i) (-1)^i, {i, 0, n}], {n, 0, 20}]  (* Geoffrey Critzer, Jan 02 2013, and adapted to the extension by Stefano Spezia, Nov 15 2018 *)

Table[Length[Select[Permutations[Join[Range[n], Range[n]]], !MatchQ[#, {___, x_, x_, ___}]&]], {n, 0, 5}] (* Gus Wiseman, Jul 04 2020 *)


(PARI) vector(20, n, sum(k=0, n, binomial(n, k)*(-1)^(n-k)*(n+k)!/2^k)) \\ Michel Marcus, Aug 10 2015

(MAGMA) I:=[0, 2]; [n le 2 select I[n] else n*(2*n-1)*Self(n-1) + (n-1)*n*Self(n-2): n in [1..20]]; // Vincenzo Librandi, Aug 10 2015


Cf. A114939 = preferred seating arrangements of n couples.

Cf. A007060 = arrangements of n couples with no adjacent spouses; A007060(n) = 2^n * A114938(n) (this sequence).

Cf. A193638.

Cf. A278990 = number of loopless linear chord diagrams with n chords.

Cf. A000806 = Bessel polynomial y_n(-1).

The version for multisets with prescribed multiplicities is A335125.

The version for prime indices is A335452.

Anti-run compositions are counted by A003242.

Anti-run compositions are ranked by A333489.

Inseparable partitions are counted by A325535.

Inseparable partitions are ranked by A335448.

Separable partitions are counted by A325534.

Separable partitions are ranked by A335433.

Cf. A007716, A128695, A292884, A335126, A335434, A335451.

Other sequences involving the multiset {1,1,2,2,...,n,n}: A001147, A007717, A020555, A094574, A316972.

Sequence in context: A013525 A270531 A229781 * A082653 A332231 A274389

Adjacent sequences:  A114935 A114936 A114937 * A114939 A114940 A114941




Hugo Pfoertner, Jan 08 2006


a(0)=1 prepended by Seiichi Manyama, Nov 15 2018



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Last modified December 1 05:33 EST 2021. Contains 349426 sequences. (Running on oeis4.)