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A068051
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Number of n-node connected graphs with one cycle, possibly of length 1 or 2.
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5
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1, 2, 4, 9, 20, 49, 118, 300, 765, 1998, 5255, 14027, 37670, 102095, 278262, 763022, 2101905, 5816142, 16153148, 45017423, 125836711, 352723949, 991143727, 2791422887, 7877935985, 22275473767, 63096075118, 179012076933
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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MATHEMATICA
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nn=20; t[x_]:=Sum[a[n]x^n, {n, 0, nn}]; sol=SolveAlways[0==Series[t[x]-x Product[1/(1-x^i)^a[i], {i, 1, nn}], {x, 0, nn}], x]; b=Table[a[n], {n, 1, nn}]/.sol//Flatten; Map[Total, Drop[Transpose[Table[Take[CoefficientList[CycleIndex[DihedralGroup[n], s]/.Table[s[j]->Table[Sum[b[[i]]x^(i*k), {i, 1, nn}], {k, 1, nn}][[j]], {j, 1, n}], x], nn], {n, 1, nn}]], 1]] (* Geoffrey Critzer, Mar 24 2013 *)
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PROG
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(PARI) \\ TreeGf gives gf of A000081
TreeGf(N)={my(A=vector(N, j, 1)); for(n=1, N-1, A[n+1] = 1/n * sum(k=1, n, sumdiv(k, d, d*A[d]) * A[n-k+1] ) ); x*Ser(A)}
seq(n)={my(t=TreeGf(n)); my(g(e)=subst(t + O(x*x^(n\e)), x, x^e) + O(x*x^n)); Vec((sum(d=1, n, eulerphi(d)/d*log(1/(1-g(d)))) + ((1+g(1))^2/(1-g(2))-1)/2)/2)} \\ Andrew Howroyd, Jun 20 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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