OFFSET
0,4
COMMENTS
The partial sums give the number of unlabeled graphs on n nodes of degree 0, 1, or 2.
REFERENCES
Herbert S. Wilf, Generatingfunctiontology, Academic Press, p. 106.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..1000
FORMULA
O.g.f.: (1/(1-x^2)) * Product_{i>=3} 1/(1-x^i)^2.
MATHEMATICA
CoefficientList[Series[1/(1-x^2) Product[1/(1-x^i)^2, {i, 3, 20}], {x, 0, 20}], x]
PROG
(PARI) seq(n)={Vec(prod(i=3, n, 1/(1-x^i)^2 + O(x*x^n))/(1-x^2))} \\ Andrew Howroyd, Oct 20 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Geoffrey Critzer, Feb 21 2011
EXTENSIONS
Terms a(21) and beyond from Andrew Howroyd, Oct 20 2018
STATUS
approved