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 A054369 Number of unlabeled 7-ary cacti having n polygons. 4
 1, 1, 7, 28, 231, 2100, 23884, 285390, 3626295, 47813815, 650367788, 9066061200, 128987761308, 1866877313448, 27417589615234, 407771633434368, 6131640607962135, 93096368350684727, 1425633586192690945, 21998953427963954554, 341803227016091180620 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 REFERENCES Miklos Bona, Michel Bousquet, Gilbert Labelle, Pierre Leroux, Enumeration of m-ary cacti, Advances in Applied Mathematics, 24 (2000), 22-56. LINKS Andrew Howroyd, Table of n, a(n) for n = 0..200 FORMULA a(n) = (1/n)*(Sum_{d|n} phi(n/d)*binomial(7*d, d)) - 6*binomial(7*n, n)/(6*n+1) for n > 0. - Andrew Howroyd, May 02 2018 a(n) ~ 7^(7*n + 1/2) / (2 * sqrt(3*Pi) * n^(5/2) * 6^(6*n + 1)). - Vaclav Kotesovec, Jul 17 2017 MATHEMATICA a[n_] := If[n == 0, 1, (Binomial[7*n, n]/(6 n + 1) + DivisorSum[n, Binomial[7*#, #]*EulerPhi[n/#]*Boole[# < n] & ])/n]; Table[a[n], {n, 0, 20}] (* Jean-François Alcover, Jul 17 2017 *) PROG (PARI) a(n) = if(n==0, 1, sumdiv(n, d, eulerphi(n/d)*binomial(7*d, d))/n - 6*binomial(7*n, n)/(6*n+1)) \\ Andrew Howroyd, May 02 2018 CROSSREFS Column k=7 of A303912. Cf. A054370, A054371. Sequence in context: A224663 A203296 A058822 * A185360 A198028 A244300 Adjacent sequences:  A054366 A054367 A054368 * A054370 A054371 A054372 KEYWORD nonn AUTHOR EXTENSIONS More terms from Jean-François Alcover, Jul 17 2017 STATUS approved

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Last modified April 6 15:23 EDT 2020. Contains 333276 sequences. (Running on oeis4.)