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A004110 Number of n-node unlabeled graphs without endpoints (i.e., no nodes of degree 1).
(Formerly M1504)
21
1, 1, 1, 2, 5, 16, 78, 588, 8047, 205914, 10014882, 912908876, 154636289460, 48597794716736, 28412296651708628, 31024938435794151088, 63533059372622888758054, 244916078509480823407040988, 1783406527599529094009748567708 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

a(n) is also the number of unlabeled mating graphs with n nodes. A mating graph has no two vertices with identical sets of neighbors. - Tanya Khovanova, Oct 23 2008

REFERENCES

F. Harary, Graph Theory, Wiley, 1969. See illustrations in Appendix 1.

F. Harary and E. Palmer, Graphical Enumeration, (1973), compare formula (8.7.11).

R. W. Robinson, personal communication.

R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1976.

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

LINKS

Andrew Howroyd, Table of n, a(n) for n = 0..50 (terms 0..26 from R. W. Robinson)

David Cook II, Nested colourings of graphs, arXiv preprint arXiv:1306.0140 [math.CO], 2013.

Ji Li, Ji Li's Research page [archived copy]

R. J. Mathar, Illustrations for n=1..5 nodes

R. W. Robinson, Graphs without endpoints - computer printout

N. J. A. Sloane, Illustration of a(0)-a(5)

MATHEMATICA

permcount[v_] := Module[{m=1, s=0, k=0, t}, For[i=1, i <= Length[v], i++, t = v[[i]]; k = If[i>1 && t == v[[i-1]], k+1, 1]; m *= t*k; s += t]; s!/m];

edges[v_] := Sum[GCD[v[[i]], v[[j]]], {i, 2, Length[v]}, {j, 1, i - 1}] + Total[Quotient[v, 2]];

a[n_] := Sum[permcount[p]*2^edges[p]*Coefficient[Product[1-x^p[[i]], {i, 1, Length[p]}], x, n-k]/k!, {k, 1, n}, {p, IntegerPartitions[k]}]; a[0] = 1;

Table[a[n], {n, 0, 18}] (* Jean-Fran├žois Alcover, Oct 27 2018, after Andrew Howroyd *)

PROG

(PARI) \\ Compare A000088.

permcount(v) = {my(m=1, s=0, k=0, t); for(i=1, #v, t=v[i]; k=if(i>1&&t==v[i-1], k+1, 1); m*=t*k; s+=t); s!/m}

edges(v) = {sum(i=2, #v, sum(j=1, i-1, gcd(v[i], v[j]))) + sum(i=1, #v, v[i]\2)}

a(n) = {my(s=0); sum(k=1, n, forpart(p=k, s+=permcount(p) * 2^edges(p) * polcoef(prod(i=1, #p, 1-x^p[i]), n-k)/k!)); s} \\ Andrew Howroyd, Sep 09 2018

CROSSREFS

Cf. A059166 (n-node connected labeled graphs without endpoints), A059167 (n-node labeled graphs without endpoints), A004108 (n-node connected unlabeled graphs without endpoints), A006024 (number of labeled mating graphs with n nodes).

If isolated nodes are forbidden, see A261919.

Sequence in context: A263914 A218168 A054960 * A236960 A290609 A048754

Adjacent sequences:  A004107 A004108 A004109 * A004111 A004112 A004113

KEYWORD

nonn

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified November 17 12:09 EST 2018. Contains 317276 sequences. (Running on oeis4.)