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A038205
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Number of derangements of n where minimal cycle size is at least 3.
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110
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1, 0, 0, 2, 6, 24, 160, 1140, 8988, 80864, 809856, 8907480, 106877320, 1389428832, 19452141696, 291781655984, 4668504894480, 79364592318720, 1428562679845888, 27142690734936864, 542853814536802656, 11399930109077490560, 250798462399300784640
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OFFSET
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0,4
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COMMENTS
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Permutations with no cycles of length 1 or 2.
Related to (and bounded by) "derangements" (A000166). Minimal cycle size 3 is interesting because of its physical analog. Consider a fully-connected network of n nodes where the objects stored at the nodes must derange but can't do so in such a way that any two objects would collide along the connecting "pipe" between their nodes.
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REFERENCES
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G. Paquin, Dénombrement de multigraphes enrichis, Mémoire, Math. Dept., Univ. Québec à Montréal, 2004.
H. S. Wilf, Generatingfunctionology, Academic Press, NY, 1990, p. 147, Eq. 5.2.9 (q=2).
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LINKS
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FORMULA
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a(n) = Sum_{i=3..n} binomial(n-1,i-1) * (i-1)! * a(n-i).
E.g.f.: exp(-x-x^2/2)/(1-x) = exp( Sum_{k>2} x^k / k ).
a(n) = n! * Sum_{m=1..n} ((Sum_{k=0..m} k!*(-1)^(m-k) *binomial(m,k) *Sum_{i=0..n-m} abs(stirling1(i+k,k)) *binomial(m-k,n-m-i) *2^(-n+m+i) /(i+k)!))/m!; a(0)=1. - Vladimir Kruchinin, Feb 01 2011
a(n) = (n-1)*a(n-1) + (n-1)*(n-2)*a(n-3). - Peter Bala, Apr 18 2012
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EXAMPLE
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a(5) = 24 because, with a minimum cycle size of 3, the only way to derange all 5 elements is to have them move around in one large 5-cycle. The number of possible moves is (5-1)! = 4! = 24.
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MAPLE
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with(combstruct): ZL2:=[S, {S=Set(Cycle(Z, card>2))}, labeled] :seq(count(ZL2, size=n), n=0..21); # Zerinvary Lajos, Sep 26 2007
with(combstruct):a:=proc(m) [ZZ, {ZZ=Set(Cycle(Z, card>m))}, labeled]; end: A038205:=a(2):seq(count(A038205, size=n), n=0..21); # Zerinvary Lajos, Oct 02 2007
G:= exp(-x-x^2/2)/(1-x): Gser:=series(G, x, 26): a:= n-> n!*coeff(Gser, x, n): seq(a(n), n=0..25); # Paul Weisenhorn, May 29 2010
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MATHEMATICA
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PROG
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(PARI) x='x+O('x^30); Vec(serlaplace(exp(-x-x^2/2)/(1-x))) \\ G. C. Greubel, Jun 25 2018
(Magma) m:=30; R<x>:=PowerSeriesRing(Rationals(), m); b:=Coefficients(R!(Exp(-x-x^2/2)/(1-x))); [Factorial(n-1)*b[n]: n in [1..m]]; // G. C. Greubel, Jun 25 2018
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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