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A069840
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Number of different (unlabeled) 2-cell embeddings of the n-wheel graph W_(n+1) on n+1 nodes into orientable surfaces.
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1
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16, 80, 666, 6588, 80886, 1146916, 18583160, 337808300, 6812539360, 150922350288, 3643698427650, 95221941543232, 2678117152113428, 80658585770586368, 2590036811212597862, 88333886984966359596, 3188853320209605353376, 121480126482182314239216, 4870248707151384381179450
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OFFSET
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4,1
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COMMENTS
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Values of a(n) for n <= 3 are not well-defined.
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LINKS
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FORMULA
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a(n)=1/(2*n)*sum_(d|n) phi(d)^2*2^(n/d)*(n/d-1)!*d^(n/d-1), n odd; a(n)=1/(2*n)*sum_(d|n) phi(d)^2*2^(n/d)*(n/d-1)!*d^(n/d-1)+ 2^(n-3)*(n/2-1)!, n even, where phi(n) is the Euler totient function A000010.
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MATHEMATICA
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f[n_] := Block[{d = Divisors[n], s}, s = Apply[Plus, EulerPhi[d]^2*2^(n/d)*(n/d - 1)!*d^(n/d - 1)]/(2n); If[ EvenQ[n], s = s + 2^(n - 3)*(n/2 - 1)! ]; s];
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PROG
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(PARI) a(n) = 1/(2*n)*sumdiv(n, d, eulerphi(d)^2*2^(n/d)*(n/d-1)!*d^(n/d-1)) + if (!(n % 2), 2^(n-3)*(n/2-1)!); \\ Michel Marcus, May 30 2019
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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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