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A054358
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Number of unlabeled asymmetric 2-ary cacti having n polygons.
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3
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1, 1, 0, 1, 2, 8, 18, 61, 170, 538, 1654, 5344, 17252, 57146, 190786, 646305, 2209050, 7626164, 26532732, 93013852, 328196780, 1165060170, 4158266282, 14915635376, 53745892932, 194477856048, 706436256598, 2575316698792, 9419568272632
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OFFSET
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0,5
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LINKS
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Miklos Bona, Michel Bousquet, Gilbert Labelle, and Pierre Leroux, Enumeration of m-ary cacti, Advances in Applied Mathematics, 24 (2000), 22-56.
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FORMULA
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a(n) = (1/n)*(Sum_{d|n} mu(n/d)*binomial(2*d, d)) - binomial(2*n, n)/(n+1) for n > 0. - Andrew Howroyd, May 02 2018
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MATHEMATICA
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a[0] = 1; a[n_] := DivisorSum[n, MoebiusMu[n/#] Binomial[2#, #]&]/n - Binomial[2n, n]/(n+1);
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PROG
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(PARI) a(n) = if(n==0, 1, sumdiv(n, d, moebius(n/d)*binomial(2*d, d))/n - binomial(2*n, n)/(n+1)) \\ Andrew Howroyd, May 02 2018
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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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