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 A059167 Number of n-node labeled graphs without endpoints. 28
 1, 1, 1, 2, 15, 314, 13757, 1142968, 178281041, 52610850316, 29702573255587, 32446427369694348, 69254848513798160815, 291053505824567573585744, 2421830049319361003822380177, 40050220743831370293688592267252 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 REFERENCES F. Harary and E. Palmer, Graphical Enumeration, (1973), p. 31, problem 1.16(a). LINKS Robert Israel, Table of n, a(n) for n = 0..81 Marko R. Riedel, Geoffrey Critzer, Math.Stackexchange.com, Proof of the closed form of the e.g.f. by combinatorial species. - Marko Riedel, Sep 18 2016 FORMULA a(n) = Sum_{i=0..n-1} binomial(n-1, i)*b(i+1)*a(n-i-1), n>0, a(0)=1, where b(n) is number of n-node connected labeled graphs without endpoints (Cf. A059166). E.g.f.: exp(x^2/2)*(Sum_{n >= 0} 2^binomial(n, 2)*(x/exp(x))^n/n!). - Vladeta Jovovic, Mar 23 2004 a(n) ~ 2^(n*(n-1)/2). - Vaclav Kotesovec, Sep 22 2016 MAPLE F:= proc(N) local S;    S:= series(exp(1/2*x^2)*Sum(2^binomial(n, 2)*(x/exp(x))^n/n!, n = 0 .. N), x, N+1);    seq(coeff(S, x, i)*i!, i=0..N) end proc: F(20); # Robert Israel, Sep 18 2016 MATHEMATICA b[n_] := If[n < 3, Boole[n == 1], n!*Sum[(-1)^(n - j) * SeriesCoefficient[1 + Log[Sum[2^(k*(k - 1)/2)*x^k/k!, {k, 0, j}]], {x, 0, j}] * j^(n - j)/(n - j)!, {j, 0, n}]]; a[0] = 1; a[n_] := a[n] = Sum[Binomial[n - 1, i] b[i + 1] a[n - i - 1], {i, 0, n - 1}]; Table[a@ n, {n, 0, 15}] (* Michael De Vlieger, Sep 19 2016, after Vaclav Kotesovec at A059166 *) PROG (PARI) seq(n)={my(A=x/exp(x + O(x^n))); Vec(serlaplace(exp(x^2/2 + O(x*x^n)) * sum(k=0, n, 2^binomial(k, 2)*A^k/k!)))} \\ Andrew Howroyd, Sep 09 2018 CROSSREFS Cf. A059166 (n-node connected labeled graphs without endpoints), A004108 (n-node connected unlabeled graphs without endpoints), A004110 (n-node unlabeled graphs without endpoints). Sequence in context: A231256 A304120 A255929 * A003025 A015200 A331344 Adjacent sequences:  A059164 A059165 A059166 * A059168 A059169 A059170 KEYWORD easy,nonn AUTHOR Vladeta Jovovic, Jan 12 2001 EXTENSIONS More terms from John Renze (jrenze(AT)yahoo.com), Feb 01 2001 STATUS approved

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Last modified August 11 18:32 EDT 2020. Contains 336428 sequences. (Running on oeis4.)