A005703 - OEIS

A005703

Number of n-node connected graphs with at most one cycle.
(Formerly M1151)

1, 1, 1, 2, 4, 8, 19, 44, 112, 287, 763, 2041, 5577, 15300, 42419, 118122, 330785, 929469, 2621272, 7411706, 21010378, 59682057, 169859257, 484234165, 1382567947, 3952860475, 11315775161, 32430737380, 93044797486, 267211342954, 768096496093, 2209772802169

(list; graph; refs; listen; history; text; internal format)

OFFSET

0,4

COMMENTS

a(n) is the number of pseudotrees on n nodes. - Eric W. Weisstein, Jun 11 2012

Also unlabeled connected graphs covering n vertices with at most n edges. For this definition we have a(1) = 0 and possibly a(0) = 0. - Gus Wiseman, Feb 20 2024

REFERENCES

J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 150.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Washington Bomfim, Table of n, a(n) for n = 0..500

R. K. Guy, Letters to N. J. A. Sloane and J. W. Moon, 1988

Richard J. Mathar, Counting Connected Graphs without Overlapping Cycles, arXiv:1808.06264 [math.CO], 2018.

Eric Weisstein's World of Mathematics, Pseudotree

Wikipedia, Pseudoforest

FORMULA

a(n) = A000055(n) + A001429(n).

EXAMPLE

From Gus Wiseman, Feb 20 2024: (Start)

Representatives of the a(0) = 1 through a(5) = 8 graphs:

{} . {12} {12,13} {12,13,14} {12,13,14,15}

{12,13,23} {12,13,24} {12,13,14,25}

{12,13,14,23} {12,13,24,35}

{12,13,24,34} {12,13,14,15,23}

{12,13,14,23,25}

{12,13,14,23,45}

{12,13,14,25,35}

{12,13,24,35,45}

(End)

MATHEMATICA

Needs["Combinatorica`"]; nn = 20; t[x_] := Sum[a[n] x^n, {n, 1, nn}];

a[0] = 0;

b = Drop[Flatten[

sol = SolveAlways[

0 == Series[

t[x] - x Product[1/(1 - x^i)^a[i], {i, 1, nn}], {x, 0, nn}],

x]; Table[a[n], {n, 0, nn}] /. sol], 1];

r[x_] := Sum[b[[n]] x^n, {n, 1, nn}]; c =

Drop[Table[

CoefficientList[

Series[CycleIndex[DihedralGroup[n], s] /.

Table[s[i] -> r[x^i], {i, 1, n}], {x, 0, nn}], x], {n, 3,

nn}] // Total, 1];

d[x_] := Sum[c[[n]] x^n, {n, 1, nn}]; CoefficientList[

Series[r[x] - (r[x]^2 - r[x^2])/2 + d[x] + 1, {x, 0, nn}], x] (* Geoffrey Critzer, Nov 17 2014 *)

PROG

(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(1 + g(1) + (g(2) - g(1)^2)/2 + sum(k=3, n, sumdiv(k, d, eulerphi(d)*g(d)^(k/d))/k + if(k%2, g(1)*g(2)^(k\2), (g(1)^2+g(2))*g(2)^(k/2-1)/2))/2)}; \\ Andrew Howroyd and Washington Bomfim, May 15 2021

CROSSREFS

Cf. A000055, A000081, A001429 (labeled A057500), A134964 (number of pseudoforests, labeled A133686).

The labeled version is A129271.

The connected complement is A140636, labeled A140638.

Non-connected: A368834 (labeled A367869) or A370316 (labeled A369191).

A001187 counts connected graphs, unlabeled A001349.

A006125 counts simple graphs, unlabeled A000088.

A006129 counts covering graphs, unlabeled A002494.

A062734 counts connected graphs by number of edges.