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 A005649 Expansion of e.g.f. (2 - e^x)^(-2). (Formerly M1866) 75
 1, 2, 8, 44, 308, 2612, 25988, 296564, 3816548, 54667412, 862440068, 14857100084, 277474957988, 5584100659412, 120462266974148, 2772968936479604, 67843210855558628, 1757952715142990612, 48093560991292628228, 1385244691781856307124 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Exponential self-convolution of numbers of preferential arrangements. Number of compatible bipartitional relations on a set of cardinality n. - Ralf Stephan, Apr 27 2003 Stirling transform of A000142, shifted left one place: 1, 2, 6, 24, 120, 720, ... - Philippe Deléham, May 17 2005; corrected by Ilya Gutkovskiy, Jul 25 2018 With an extra 1 at the beginning, coefficients of the formal (divergent) series expansion at infinity of Sum_{k>=0} 1/binomial(x,k) = 1+1/x+2/x^2+8/x^3+... Also Sum_{k>=0} k!/x^k Product_{i=1..k-1} 1/(1-i/x) yields a generating function in 1/x. - Roland Bacher, Nov 21 2000 Stirling-Bernoulli transform of A001057: 1, -1, 2, -2, 3, -3, 4, ... - Philippe Deléham, May 27 2015 a(n) is the total number of open sets summed over all chain topologies that can be placed on an n-set. A chain topology is a topology whose open sets can be totally ordered by inclusion. - Geoffrey Critzer, Apr 06 2017 From Gus Wiseman, Jun 10 2020: (Start) Also the number of length n + 1 sequences covering an initial interval of positive integers with no adjacent equal parts (anti-runs). For example, the a(0) = 1 through a(2) = 8 anti-runs are: (1) (1,2) (1,2,1) (2,1) (1,2,3) (1,3,2) (2,1,2) (2,1,3) (2,3,1) (3,1,2) (3,2,1) Also the number of ordered set partitions of {1,...,n + 1} with no two successive vertices in the same block. For example, the a(0) = 1 through a(2) = 8 ordered set partitions are: {{1}} {{1},{2}} {{1,3},{2}} {{2},{1}} {{2},{1,3}} {{1},{2},{3}} {{1},{3},{2}} {{2},{1},{3}} {{2},{3},{1}} {{3},{1},{2}} {{3},{2},{1}} (End) REFERENCES L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 294. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Alois P. Heinz, Table of n, a(n) for n = 0..423 (first 101 terms from T. D. Noe) José A. Adell, Beáta Bényi, Venkat Murali, and Sithembele Nkonkobe, Generalized Barred Preferential Arrangements, Transactions on Combinatorics (2022). Connor Ahlbach, Jeremy Usatine and Nicholas Pippenger, Barred Preferential Arrangements, Electron. J. Combin., Volume 20, Issue 2 (2013), #P55. D. Foata, and C. Krattenthaler, Graphical Major Indices, II, Séminaire Lotharingien de Combinatoire, B34k, 16 pp., 1995. D. Foata and D. Zeilberger, The Graphical Major Index, arXiv:math/9406220 [math.CO], 1994. INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 154 Augustine O. Munagi, Two Applications of the Bijection on Fibonacci Set Partitions, Fibonacci Quart. 55 (2017), no. 5, 144-148. FORMULA E.g.f.: 1/(2-exp(x))^2. a(n) = (A000670(n) + A000670(n+1)) / 2. - Philippe Deléham, May 16 2005 a(n) = D^n(1/(1-x)^2) evaluated at x = 0, where D is the operator (1+x)*d/dx. Cf. A000670 and A052841. - Peter Bala, Nov 25 2011 E.g.f.: 1/(2-exp(x))^2 = 1/(G(0) + 4), G(k) = 1-4/((2^k)-x*(4^k)/((2^k)*x-(2*k+2)/G(k+1))); (continued fraction). - Sergei N. Gladkovskii, Dec 15 2011 O.g.f.: Sum_{n>=0} (2*n)!/n! * x^n / Product_{k=1..n} (1 + (n+k)*x). - Paul D. Hanna, Jan 03 2013 G.f.: (G(0) - 1)/(x-1) where G(k) = 1 - (k+1)/(1-k*x)/(1-x/(x-1/G(k+1) )); (recursively defined continued fraction). - Sergei N. Gladkovskii, Jan 15 2013 G.f.: 1/G(0) where G(k) = 1 - x*(k+2)/( 1 - 2*x*(k+1)/G(k+1) ); (continued fraction ). - Sergei N. Gladkovskii, Mar 23 2013 a(n) = Sum_{k = 0..n} A163626(n,k) * A001057(k+1). - Philippe Deléham, May 27 2015 a(n) ~ n! * n / (4 * (log(2))^(n+2)). - Vaclav Kotesovec, Jul 01 2018 a(n) = Sum_{k=0..n} Stirling2(n,k)*(k + 1)!. - Ilya Gutkovskiy, Jul 25 2018 MAPLE b:= proc(n, m) option remember; `if`(n=0, (m+1)!, m*b(n-1, m)+b(n-1, m+1)) end: a:= n-> b(n, 0): seq(a(n), n=0..23); # Alois P. Heinz, Aug 03 2021 MATHEMATICA f[n_] := Sum[(i + j)^n/2^(2 + i + j), {i, 0, Infinity}, {j, 0, Infinity}]; Array[f, 20, 0] (* Vladimir Reshetnikov, Dec 31 2008 *) a[n_] := (-1)^n (PolyLog[-n-1, 2] - PolyLog[-n, 2])/4; Array[f, 20, 0] (* Vladimir Reshetnikov, Jan 23 2011 *) Range[0, 19]! CoefficientList[Series[(2 - Exp@ x)^-2, {x, 0, 19}], x] (* Robert G. Wilson v, Jan 23 2011 *) nn = 19; Range[0, nn]! CoefficientList[Series[1 + D[u^2 (Exp[z] - 1)/(1 - u (Exp[z] - 1)), u] /. u -> 1, {z, 0, nn}], z] (* Geoffrey Critzer, Apr 06 2017 *) allnorm[n_]:=If[n<=0, {{}}, Function[s, Array[Count[s, y_/; y<=#]+1&, n]]/@Subsets[Range[n-1]+1]]; Table[Length[Select[Join@@Permutations/@allnorm[n], FreeQ[Differences[#], 0]&]], {n, 0, 6}] (* Gus Wiseman, Jun 10 2020 *) With[{nn=20}, CoefficientList[Series[1/(2-E^x)^2, {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Oct 02 2021 *) PROG (PARI) a(n)=if(n<0, 0, n!*polcoeff(subst(1/(1-y)^2, y, exp(x+x*O(x^n))-1), n)) (PARI) a(n)=polcoeff(sum(m=0, n, (2*m)!/m!*x^m/prod(k=1, m, 1+(m+k)*x+x*O(x^n))), n) for(n=0, 20, print1(a(n), ", ")) \\ Paul D. Hanna, Jan 03 2013 (Maxima) t(n):=sum(stirling2(n, k)*k!, k, 0, n); makelist(sum(binomial(n, k)*t(k)*t(n-k), k, 0, n), n, 0, 20); \\ Emanuele Munarini, Oct 02 2012 CROSSREFS Cf. A000670. 2*A083410(n)=a(n), if n>0. Pairwise sums of A052841 and also of A089677. Anti-run compositions are counted by A003242. A triangle counting maximal anti-runs of compositions is A106356. Anti-runs of standard compositions are counted by A333381. Adjacent unequal pairs in standard compositions are counted by A333382. Cf. A000110, A008277, A055932, A124762, A238279, A238424, A317081. Sequence in context: A137984 A191810 A172109 * A253950 A212913 A321628 Adjacent sequences: A005646 A005647 A005648 * A005650 A005651 A005652 KEYWORD nonn,easy,nice AUTHOR STATUS approved

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Last modified December 3 11:24 EST 2022. Contains 358517 sequences. (Running on oeis4.)