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A208184
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Number of distinct n-colored necklaces with 3 beads per color.
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2
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1, 1, 4, 188, 30804, 11211216, 7623616080, 8690922240480, 15391623287043360, 40018220546304026880, 146226577876194816241920, 725283826265926287362419200, 4746982642910487550771226611200, 40045545575592872978305843519334400
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) = Sum_{d|3} phi(3/d)*(n*d)!/(d!^n*n*3) if n>0 and a(0) = 1.
For n > 0, a(n) = (3*n)!/(3*n*6^n) + 2*(n-1)!/3. - Vaclav Kotesovec, Aug 23 2015
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EXAMPLE
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a(0) = 1: the empty necklace.
a(1) = 1: {000}.
a(2) = 4: {000111, 001011, 010011, 010101}.
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MAPLE
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with(numtheory);
a:= n-> `if`(n=0, 1, add(phi(3/d) *(n*d)!/(d!^n *3*n), d={1, 3})):
seq(a(n), n=0..20);
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MATHEMATICA
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Flatten[{1, Table[(3*n)!/(3*n*6^n) + 2*(n-1)!/3, {n, 1, 20}]}] (* Vaclav Kotesovec, Aug 23 2015 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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