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 A193638 Number of permutations of the multiset {1,1,1,2,2,2,3,3,3,...,n,n,n} with no two consecutive terms equal. 2
 1, 0, 2, 174, 41304, 19606320, 16438575600, 22278418248240, 45718006789687680, 135143407245840698880, 553269523327347306412800, 3039044104423605600086688000, 21819823367694505460651694873600, 200345011881335747639978525387827200 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Andrew Woods, Table of n, a(n) for n = 0..101 H. Eriksson, A. Martin, Enumeration of Carlitz multipermutations, arXiv:1702.04177 [math.CO], 2017. FORMULA a(n) = A190826(n) * n! for n >= 1. a(n) = Sum_{s+t+u=n}((-1)^t*multinomial(n;s,t,u)*(3s+2t+u)!/(3!)^s. - Alexis Martin, Nov 16 2017. EXAMPLE a(2) = 2 because there are two permutations of {1,1,1,2,2,2} avoiding equal consecutive terms: 121212 and 212121. MAPLE a:= proc(n) option remember; `if`(n<3, (n-1)*(3*n-2)/2,       n*((3*n-1)*(3*n^2-5*n+4) *a(n-1)       +2*(n-1)*(6*n^2-9*n-1) *a(n-2)       -4*n*(n-1)*(n-2) *a(n-3))/(2*n-2))     end: seq(a(n), n=0..20);  # Alois P. Heinz, Jun 05 2013 MATHEMATICA a[n_] := 1/6^n Sum[(n+j)! Sum[Binomial[n, k] Binomial[2k, j] (-3)^(n+k-j), {k, Ceiling[j/2], n}], {j, 0, 2n}];  Table[a[n], {n, 0, 13}] (* Jean-François Alcover, Jul 22 2017, after Tani Akinari *) PROG (Maxima) a(n):= (1/6^n)*sum((n+j)!*sum(binomial(n, k)*binomial(2*k, j)* (-3)^(n+k-j), k, ceiling(j/2), n), j, 0, 2*n); [Tani Akinari, Sep 23 2012] (Python) class Memoize:     def __init__(self, func):         self.func=func         self.cache={}     def __call__(self, arg):         if arg not in self.cache:             self.cache[arg] = self.func(arg)         return self.cache[arg] @Memoize def a(n): return (n - 1)*(3*n - 2)/2 if n<3 else n*((3*n - 1)*(3*n**2 - 5*n + 4)*a(n - 1) + 2*(n - 1)*(6*n**2 - 9*n - 1)*a(n - 2) - 4*n*(n - 1)*(n - 2)*a(n - 3))/(2*n - 2) print map(a, xrange(51)) # Indranil Ghosh, Jul 22 2017, formula after Maple code CROSSREFS Cf. A114938 = similar with two copies instead of three. Cf. A193624 = arrangements of triplets with no adjacent siblings; A193624(n) = 6^n * a(n). Sequence in context: A139935 A281958 A172231 * A215123 A219724 A103427 Adjacent sequences:  A193635 A193636 A193637 * A193639 A193640 A193641 KEYWORD nonn AUTHOR Andrew Woods, Aug 01 2011 STATUS approved

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Last modified March 22 04:12 EDT 2018. Contains 301047 sequences. (Running on oeis4.)