A054370 Number of unlabeled asymmetric 7-ary cacti having n polygons.

1, 1, 0, 21, 182, 2093, 23394, 285383, 3621150, 47813367, 650302814, 9066061193, 128986881660, 1866877313441, 27417577343674, 407771633368995, 6131640430397790, 93096368350684720, 1425633583562073228, 21998953427963954547, 341803226976420690600

Offset: 0

Author

Simon Plouffe

Keywords

• nonn

Links

• Andrew Howroyd, Table of n, a(n) for n = 0..200
• Miklos Bona, Michel Bousquet, Gilbert Labelle, and Pierre Leroux, Enumeration of m-ary cacti, Advances in Applied Mathematics, 24 (2000), 22-56.
• Index entries for sequences related to cacti

Crossrefs

Column k=7 of A303913.

Cf. A054369, A054371.

Programs

Mathematica

a[0] = 1;
a[n_] := DivisorSum[n, MoebiusMu[n/#] Binomial[7#, #]&]/n - 6 Binomial[7n, n]/(6n + 1);
Table[a[n], {n, 0, 20}] (* Jean-François Alcover, Jul 01 2018, after Andrew Howroyd *)

PARI

a(n) = if(n==0, 1, sumdiv(n, d, moebius(n/d)*binomial(7*d, d))/n - 6*binomial(7*n, n)/(6*n+1)) \\ Andrew Howroyd, May 02 2018

Formula

a(n) = (1/n)*(Sum_{d|n} mu(n/d)*binomial(7*d, d)) - 6*binomial(7*n, n)/(6*n+1) for n > 0. - Andrew Howroyd, May 02 2018

Extensions

Terms a(11) and beyond from Andrew Howroyd, May 02 2018