1,4

Also number of connected unicyclic graphs on n labeled nodes. - Vladeta Jovovic (vladeta(AT)Eunet.yu), Oct 26 2004

a(n) is the number of trees on vertex set [n] = {1,2,...,n} rooted at 1 with one marked inversion (an inversion is a pair (i,j) with i>j and j a descendant of i in the tree). Here is a bijection from the title graphs (on [n]) to these marked trees. A title graph has exactly one cycle. There is a unique path from vertex 1 to this cycle, first meeting it at k, say (k may = 1). Let i and j be the two neighbors of k in the cycle, with i the larger of the two. Delete the edge k<->j thereby forming a tree (in which j is a descendant of i) and take (i,j) as the marked inversion. To reverse this map, create a cycle by joining the smaller element of the marked inversion to the parent of the larger element. a(n) = binom(n-1,2)A129137(n). This is because, on the above marked trees, the marked inversion is uniformly distributed over 2-element subsets of {2,3,...,n} and so a(n)/binom(n-1,2) is the number of trees on [n] (rooted at 1) for which (3,2) is an inversion. - David Callan (callan(AT)stat.wisc.edu), Mar 30 2007

F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973.

S. Janson, D. E. Knuth, T. Luczak and B. Pittel, The Birth of the Giant Component, Random Structures and Algorithms Vol. 4 (1993), 233-358.

R. J. Riddell, Contributions to the theory of condensation, Dissertation, Univ. of Michigan, Ann Arbor, 1951.

Washington G. Bomfim, Table of n, a(n) for n = 1..50

The number of labeled connected graphs with n nodes and m edges is Sum_{k=1..n} (-1)^(k+1)/k*Sum_{n_1+n_2+..n_k=n, n_i>0} n!/(Product_{i=1..k} (n_i)!)*binomial(s, m), s=Sum_{i..k} binomial(n_i, 2). - Vladeta Jovovic (vladeta(AT)Eunet.yu), Apr 10 2001

Exponential generating function: (1/2) Sum_{k=3}^\infty T(x)^k/k, with T(x)= Sum_{n=1}^\infty n^(n-1)/n! x^n. R. J. Riddell's thesis contains a closed-form expression for the number of connected graphs with m nodes and n edges. The present series applies to the special case m=n.

E.g.f.: -1/2*ln(1+LambertW(-x))+1/2*LambertW(-x)-1/4*LambertW(-x)^2.

Asymptotic expansion (with xi=sqrt(2*pi)): n^(n-1/2)*[xi/4-7/6*n^(-1/2)+xi/48*n^(-1)+131/270*n^(-3/2)+xi/1152*n^(-2)+4/2835*n^(-5/2)+O(n^(-3))]. - Keith Briggs (keith.briggs(AT)bt.com), Aug 16 2004

Row sums of A098909: a(n) = (n-1)!*n^n/2*Sum_{k=3..n} 1/(n^k*(n-k)!). - Vladeta Jovovic (vladeta(AT)Eunet.yu), Oct 26 2004

E.g. a(4)=15 because there are three different (labeled) 4-cycles, and 12 different labeled graphs with a 3-cycle and an attached, external vertex.

Cf. A000272 = labeled trees on n nodes; connected labeled graphs with n nodes and n+k edges for k=0..8: A057500 A061540 A061541 A061542 A061543 A096117 A061544 A096150 and A096224.

Cf. A001429 (unlabeled case).

Sequence in context: A091644 A027843 A027840 this_sequence A137916 A078364 A012852

Adjacent sequences: A057497 A057498 A057499 this_sequence A057501 A057502 A057503

easy,nonn

Qing-Hu Hou and David C. Torney (dct(AT)lanl.gov), Sep 01 2000

More terms and second g.f. from Vladeta Jovovic (vladeta(AT)Eunet.yu), Jul 09 2001