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 A058349 Number of connected labeled series-parallel posets on n nodes. 4
 1, 2, 12, 122, 1740, 31922, 715932, 18978122, 580513260, 20125554242, 779832497532, 33398722757402, 1566656717322060, 79879485803841362, 4398701789915269212, 260166428897541369962, 16449181879032096013740, 1107112451498156565581282, 79030557433744270179981372 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, number of labeled blobs with n edges. REFERENCES R. C. Read, Graphical enumeration by cycle-index sums: first steps toward a unified treatment, preprint, Sept. 26, 1991. R. P. Stanley, Enumeration of posets generated by disjoint unions and ordinal sums. Proc. Amer. Math. Soc. 45 (1974), 295-299 R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.39, page 133, g(n). LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..100 FORMULA Read (1991) reference gives generating functions (see PARI code for one example). A048172(n)= a(n)+A048174(n), n>1. a(n)=(n-1)!*sum(k=1..n-1, binomial(n+k-1,n-1)*sum(j=1..k, binomial(k,j)*((sum(l=0..j-1, (binomial(j,l)*((-1)^(n-l+j-1)+1)*sum(r=1..j-l, binomial(j-l,r)*2^(j-l-r-1)*(-1)^(r-j)*sum(i=0..r, (r-2*i)^(n-l+j-1)*binomial(r,i))))/(n-l+j-1)!))))), n>1, a(1)=1. - Vladimir Kruchinin, Feb 19 2012 a(n) ~ n^(n-1) / (5^(1/4)*exp(n)*(2-sqrt(5)+log((1+sqrt(5))/2))^(n-1/2)). - Vaclav Kotesovec, Mar 09 2014 MAPLE (continue from A053554) t1 := log(1+EGF053554): t2 := series(t1, x, 30); SERIESTOLISTMULT(t2); MATHEMATICA Drop[ CoefficientList[ InverseSeries[ Series[x + 2*(1 - Cosh[x]) , {x, 0, 19}], y], y], 1]* Range[19]! (* Jean-François Alcover, Sep 21 2011, after g.f. *) PROG (PARI) /* Joerg Arndt, Feb 04 2011 */ x='x+O('x^55); t=x+2*(1-cosh(x)); Vec(serlaplace(serreverse(t))) /* show terms */ (Maxima) a(n):=if n=1 then 1 else (n-1)!*sum(binomial(n+k-1, n-1)*sum(binomial(k, j)*((sum((binomial(j, l)*((-1)^(n-l+j-1)+1)*sum(binomial(j-l, r)*2^(j-l-r-1)*(-1)^(r-j)*sum((r-2*i)^(n-l+j-1)*binomial(r, i), i, 0, r), r, 1, j-l))/(n-l+j-1)!, l, 0, j-1))), j, 1, k), k, 1, n-1); [Vladimir Kruchinin, Feb 19 2012] CROSSREFS A053554(n) = A058349(n) + A058350(n) (n>=2). Sequence in context: A034524 A051782 A048173 * A013469 A246738 A039933 Adjacent sequences:  A058346 A058347 A058348 * A058350 A058351 A058352 KEYWORD nonn,easy,nice AUTHOR N. J. A. Sloane, Dec 16 2000 EXTENSIONS More terms from Joerg Arndt, Feb 04 2011 STATUS approved

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Last modified February 19 18:20 EST 2019. Contains 320321 sequences. (Running on oeis4.)