OFFSET
1,2
LINKS
Eric Weisstein's World of Mathematics, Graph Vertex.
FORMULA
a(n) = n*A000055(n).
O.g.f.: x d/dx A(x) where A(x) is the o.g.f. for A000055. - Geoffrey Critzer, Jul 06 2020
MATHEMATICA
nn = 25; f[x_] := Sum[a[n] x^n, {n, 0, nn}]; sol = SolveAlways[0 == Series[
f[x] - x Product[1/(1 - x^i)^a[i], {i, 1, nn}], {x, 0, nn}], x];
r[x_] := Sum[a[n] x^n, {n, 0, nn}] /. sol; CoefficientList[Series[x D[r[x] - 1/2 (r[x]^2 - r[x^2]), x], {x, 0, nn}], x] (* Geoffrey Critzer, Jul 06 2020 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
More terms, formula from Christian G. Bower, Jun 12 2000
STATUS
approved