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 A079146 Number of unlabeled semitransitive orders on n elements: (1+3)-free posets. 5
 1, 2, 5, 15, 49, 173, 639, 2469, 9997, 43109, 205092, 1153646, 8523086, 91156133, 1446766659, 32998508358, 1047766596136, 45632564217917, 2711308588849394, 219364550983697100, 24151476334929009951, 3618445112608409433287 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS M. Guay-Paquet, A modular relation for the chromatic symmetric functions of (3+1)-free posets, arXiv preprint arXiv:1306.2400 [math.CO], 2013. M. Guay-Paquet, A. H. Morales, E. Rowland, Structure and enumeration of (3+1)-free posets (extended abstract), arXiv:1212.5356 [math.CO], 2012. FORMULA G.f.: S(x/(1-x), T(x)), where S(x, y) is the g.f. for A221494 and T(x) is the g.f. for A221492. [Mathieu Guay-Paquet, Jan 18 2013] MATHEMATICA nmax = 23; co = Coefficient; ex = Exponent; b[n_, i_] := b[n, i] = If[n == 0, {0}, If[i<1, {}, Flatten[Table[Function[ {p}, p + j x^i] /@ b[n - i j, i - 1], {j, 0, n/i}]]]]; g[n_, k_] := g[n, k] = Sum[Sum[2^Sum[Sum[GCD[i, j] co[s, x, i] co[t, x, j], {j, 1, ex[t, x]}], {i, 1, ex[s, x]}]/Product[i^co[s, x, i]*co[s, x, i]!, {i, 1, ex[s, x]}]/Product[i^co[t, x, i] co[t, x, i]!, {i, 1, ex[t, x]}], {t, b[n + k, n + k]}], {s, b[n, n]}]; A[n_, k_] := g[Min[n, k], Abs[n - k]]; A[d_] := Sum[A[n, d - n], {n, 0, d}]; B[x_] = Sum[A[n] x^n, {n, 0, nmax}]; S[_, _] = 0; Do[S[c_, t_] = Series[1 + (c/(1 + c)) S[c, t]^2 + t S[c, t]^3, {c, 0, nmax}, {t, 0, nmax}] // Normal, {nmax}]; T[x_] = 1 - S[x/(1 - x), 1 - 2x - 1/B[x]]; Rest[CoefficientList[-T[x] + O[x]^nmax, x]] (* Jean-François Alcover, Aug 11 2018, after Alois P. Heinz *) CROSSREFS Cf. A079145 (labeled semitransitive orders), A000112. Sequence in context: A149941 A149942 A149943 * A000734 A148366 A005751 Adjacent sequences:  A079143 A079144 A079145 * A079147 A079148 A079149 KEYWORD nonn AUTHOR Detlef Pauly (dettodet(AT)yahoo.de), Dec 27 2002 EXTENSIONS More terms from Mathieu Guay-Paquet, Jan 18 2013 STATUS approved

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Last modified March 31 19:37 EDT 2020. Contains 333151 sequences. (Running on oeis4.)