OFFSET
1,2
COMMENTS
a(n) is the number of unlabeled graphs on n nodes whose connected components are a path or a cycle. - Geoffrey Critzer, Nov 28 2011
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
Herbert S. Wilf, Generatingfunctiontology, Academic Press, 1994, page 106.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
M. S. Cheema and W. E. Conway, Numerical investigation of certain asymptotic results in the theory of partitions, Math. Comp., 26 (1972), 999-1005.
FORMULA
G.f.: Product (1 - x^k)^-{c(k)}; c(k) = 1, 1, 2, 2, 2, 2, ....
MATHEMATICA
Rest[p=Product[1/(1 - x^i), {i, 1, 20}]; CoefficientList[Series[p^2 (1 - x) (1 - x^2), {x, 0, 20}], x]] (* Geoffrey Critzer, Nov 28 2011 *) (* adapted by Vincenzo Librandi, Oct 12 2017 *)
PROG
(PARI) \\ program includes a(0) = 1:
c(n) = 1 + (n>=3);
N = 66; x = 'x + O('x^N);
Vec( 1 / prod(n=1, N, (1 - x^k)^c(n)) ) \\ Joerg Arndt, Oct 12 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
More terms from Joerg Arndt, Oct 12 2017
STATUS
approved