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 A006531 Semiorders on n elements. (Formerly M3061) 8
 1, 1, 3, 19, 183, 2371, 38703, 763099, 17648823, 468603091, 14050842303, 469643495179, 17315795469063, 698171064855811, 30561156525545103, 1443380517590979259, 73161586346500098903, 3961555049961803092531, 228225249142441259147103, 13938493569348563803135339 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Labeled semiorders on n elements: (1+3) and (2+2)-free posets. - Detlef Pauly (dettodet(AT)yahoo.de), Dec 27 2002 Labeled incomplete binary trees (every vertex has a left child, a right child, neither, or both) in which every vertex with a right child but no left child has a label greater than the label of its right child. - Ira M. Gessel, Nov 09 2013 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 6.30. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..200 J. L. Chandon, J. LeMaire and J. Pouget, Dénombrement des quasi-ordres sur un ensemble fini, Math. Sci. Humaines, No. 62 (1978), 61-80. J. L. Chandon, J. LeMaire and J. Pouget, Enumeration of semiorders on a finite set, Preprint (English) of "Dénombrement des quasi-ordres sur un ensemble fini". J. L. Chandon, Letter to N. J. A. Sloane, May 1981 Julie Christophe, Jean-Paul Doignon and Samuel Fiorini, Counting Biorders, J. Integer Seqs., Vol. 6, 2003. Yan X Zhang, Four Variations on Graded Posets, arXiv preprint arXiv:1508.00318 [math.CO], 2015. FORMULA E.g.f.: C(1-exp(-x)), where C(x) = (1 - sqrt(1 - 4*x)) / (2*x) is the ordinary g.f. for the Catalan numbers A000108. [corrected by Joel B. Lewis, Mar 29 2011] a(n) = sum( S(n, k) * k! * M(k-1), k=1..n), S(n, k): Stirling number of the second kind, M(n): Motzkin number, A001006. - Detlef Pauly, Jun 06 2002 O.g.f.: Sum_{n>=1} (2*n)!/(n+1)! * x^n / Product_{k=0..n} (1+k*x). [Paul D. Hanna, Jul 20 2011] a(n) ~ n! * sqrt(3)*(log(4/3))^(1/2-n)/(sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Aug 13 2013 E.g.f.: 1/(1 + (exp(-x) - 1)/(1 + (exp(-x) - 1)/(1 + (exp(-x) - 1)/(1 + (exp(-x) - 1)/(1 + ...))))), a continued fraction. - Ilya Gutkovskiy, Nov 18 2017 From Peter Bala, Jan 15 2018: (Start) a(n) = Sum_{k = 0..n} (-1)^(n+k)*Catalan(k)*k!*Stirling2(n,k). Cf. A052895. Conjecture: for fixed k = 1,2,..., the sequence a(n) (mod k) is eventually periodic with the exact period dividing phi(k), where phi(k) is Euler's totient function A000010. For example, modulo 10 the sequence becomes (1, 1, 3, 9, 3, 1, 3, 9, 3, ...) with an apparent period 1, 3, 9, 3 of length 4 = phi(10) beginning at a(1). (End) MAPLE A006531 := n->add(stirling2(n, k)*k!*A001006(k-1), k=1..n); MATHEMATICA m[n_] := m[n] = m[n-1] + Sum[ m[k]*m[n-k-2], {k, 0, n-2}]; m[0] = a[0] = 1; a[n_] := Sum[ StirlingS2[n, k]*k!*m[k-1], {k, 1, n}]; Table[a[n], {n, 0, 17}] (* Jean-François Alcover, Jul 24 2012, after Maple *) PROG (PARI) {a(n)=polcoeff(sum(m=0, n, (2*m)!/(m+1)!*x^m/prod(k=1, m, 1+k*x+x*O(x^n))), n)} /* Paul D. Hanna, Jul 20 2011 */ CROSSREFS Cf. A000108 (unlabeled semiorders: Catalan numbers), A052895. Sequence in context: A121083 A213533 A203133 * A242369 A202617 A143633 Adjacent sequences:  A006528 A006529 A006530 * A006532 A006533 A006534 KEYWORD nonn,nice,easy AUTHOR STATUS approved

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Last modified December 14 21:22 EST 2018. Contains 318138 sequences. (Running on oeis4.)