OFFSET
0,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..50
V. A. Liskovets, Some easily derivable sequences, J. Integer Sequences, 3 (2000), #00.2.2.
FORMULA
Inverse Euler transform of A000595.
EXAMPLE
Nonisomorphic connected relations on set {1,2} are {2r1}, {1r1,2r1}, {2r1,2r2}, {1r1,2r1,2r2}, {1r2,2r1}, {1r1,1r2,2r1}, {1r1,1r2,2r1,2r2} so a(2)=7.
MATHEMATICA
nn=7; c=Join[{1, 2}, Table[CycleIndex[Join[PairGroup[SymmetricGroup[n], Ordered], Permutations[Range[n^2-n+1, n^2]], 2], s] /. Table[s[i]->2, {i, 1, n^2-n}], {n, 2, nn}]]; f[x_]:=Sum[a[n]x^n, {n, 0, nn}]; b=Sum[c[[n+1]]x^n, {n, 0, nn}]; sol=SolveAlways[b==Normal[Series[Product[1/(1-x^i)^a[i], {i, 1, nn}], {x, 0, nn}]], x]; Table[a[n], {n, 1, nn}]/.sol (* Geoffrey Critzer, Mar 31 2013 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, May 24 2000
EXTENSIONS
More terms from Vladeta Jovovic, Jul 16 2000
a(0)=1 prepended and a(13)-a(14) from Andrew Howroyd, Sep 10 2018
STATUS
approved