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A178669
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The number of permutations of [n] with 2 cycles of length 2
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1
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0, 3, 15, 45, 315, 3150, 28350, 274050, 3014550, 36330525, 472296825, 6609317715, 99139765725, 1586293008300, 26966981141100, 485404420000500, 9222683980009500, 184453709062998375
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OFFSET
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3,2
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LINKS
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FORMULA
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a(n)=n!*sum_{j=k.. [n/2]} (-1)^j/((j-k)!*2^j*k!). E.g.f. = exp(-z^2/2)*z^(2*k) / ((1-z)*2^k*k!), where k is the number of cycles of length 2.
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EXAMPLE
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a(4)=3 counts the 3 permutations (2143), (3412), (4321) with 2 cycles
of length 2
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MAPLE
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A178669 := proc(n) local k ; k :=2 ; n!*add( (-1)^j/(j-k)!/2^j/k!, j=k..n/2) ; end proc:
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MATHEMATICA
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d=Exp[-x^2/2]/(1-x); Range[0, 20]! CoefficientList[Series[(3x^4/4! )d, {x, 0, 20}], x] (* Geoffrey Critzer, Nov 29 2011 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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