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 A007334 Number of spanning trees in the graph K_{n}/e, which results from contracting an edge e in the complete graph K_{n} on n vertices (for n>=2). (Formerly M1883) 4
 1, 2, 8, 50, 432, 4802, 65536, 1062882, 20000000, 428717762, 10319560704, 275716983698, 8099130339328, 259492675781250, 9007199254740992, 336755653118801858, 13493281232954916864, 576882827135242335362, 26214400000000000000000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS The old name (referring to the Chen-Goyal article) was "[Number of] essential complementary partitions of [an] n-set." This sequence was obtained using the deletion-contraction recursions satisfied by the number of spanning trees for graphs. It is readily seen that the number of spanning trees in K_{n}-e (the complete graph K_{n} with an edge e deleted) is (n-2)*(n^{n-3}). Since the number of spanning trees in K_{n} is n^{n-2}, we see that (n-2)*(n^{n-3})+f(n)=n^{n-2} by the deletion-contraction recursion. Hence it follows that f(n)=2*n^{n-3}. - N. Eaton, W. Kook and L. Thoma (andrewk(AT)math.uri.edu), Jan 17 2004 With offset 0, the number of acyclic functions from {1,...,n} to {1,...,n+2}. See link below. - Dennis P. Walsh, Nov 27 2011 With offset 0, a(n) is the number of forests of rooted labeled trees on n nodes in which some (possibly all or none) of the trees have been specially designated. a(n) = Sum_{k=1..n} A061356(n,k)*2^k. E.g.f. is exp(T(x))^2 where T(x) is the e.g.f for A000169. The expected number of trees in each forest approaches 3 as n gets large. Cf. A225497. - Geoffrey Critzer, May 10 2013 REFERENCES J. Oxley, Matroid Theory, Oxford University Press, 1992. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS W.-K. Chen and I. C. Goyal, Tables of essential complementary partitions, IEEE Trans. Circuit Theory, 18 (1971), 562-563. W.-K. Chen and I. C. Goyal, Tables of essential complementary partitions, IEEE Trans. Circuit Theory, 18 (1971), 562-563. (Annotated scanned copy) N. Eaton, W. Kook and L. Thoma, Monotonicity for complete graphs, preprint, 2003. J.-B. Priez, A. Virmaux, Non-commutative Frobenius characteristic of generalized parking functions: Application to enumeration, arXiv:1411.4161 [math.CO], 2014-2015. Dennis Walsh, Notes on acyclic functions FORMULA a(n) = 2*n^{n-3} (n>=2). E.g.f.: (-W(-x)/x)*exp(-W(-x)). - Paul Barry, Nov 19 2010 G.f.: Sum_{n>=1} a(n+1) * x^n / (1 + n*x)^n  =  x/(1-x). - Paul D. Hanna, Jan 17 2013 EXAMPLE a(3)=2 because K_{3}/e consists of two veritices and two parallel edges, where each edge is a spanning tree. MATHEMATICA nn = 17; tx = Sum[n^(n - 1) x^n/n!, {n, 1, nn}]; Range[0, nn]! CoefficientList[Series[Exp[ tx]^2, {x, 0, nn}], x]  (* Geoffrey Critzer, May 10 2013 *) PROG (PARI) {a(n)=if(n==2, 1, 1-polcoeff(sum(k=2, n-1, a(k)*x^k/(1+(k-1)*x+x*O(x^n))^(k-1)), n))} /* Paul D. Hanna, Jan 17 2013 */ CROSSREFS The sequence is A058127(n, n-2) for n >= 2. - Peter Luschny, Apr 22 2009 Cf. A007830. Sequence in context: A225052 A295759 A089104 * A050398 A135081 A296366 Adjacent sequences:  A007331 A007332 A007333 * A007335 A007336 A007337 KEYWORD nonn AUTHOR EXTENSIONS a(6) corrected and more terms from Sean A. Irvine, Dec 19 2017 After correction, this became identical (except for the offset) with A089104, contributed by N. Eaton, W. Kook and L. Thoma (andrewk(AT)math.uri.edu), Jan 17 2004. The two entries have been merged using the older A-number. - N. J. A. Sloane, Dec 19 2017 STATUS approved

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Last modified August 7 11:08 EDT 2020. Contains 336275 sequences. (Running on oeis4.)