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 A000891 a(n) = (2*n)!*(2*n+1)! / (n! * (n+1)!)^2. 42
 1, 3, 20, 175, 1764, 19404, 226512, 2760615, 34763300, 449141836, 5924217936, 79483257308, 1081724803600, 14901311070000, 207426250094400, 2913690606794775, 41255439318353700, 588272005095043500 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Number of parallelogram polyominoes having n+1 columns and n+1 rows. - Emeric Deutsch, May 21 2003 Number of tilings of an hexagon. a(n) is the number of non-crossing partitions of [2n+1] into n+1 blocks. For example, a[1] counts 13-2, 1-23, 12-3. - David Callan, Jul 25 2005 The number of returning walks of length 2n on the upper half of a square lattice, since a(n) = Sum_{k=0..2n} binomial(2n,k)*A126120(k)*A126869(n-k). - Andrew V. Sutherland, Mar 24 2008 For sequences counting walks in the upper half-plane starting from the origin and finishing at the lattice points (0,m) see A145600 (m = 1), A145601 (m = 2), A145602 (m = 3) and A145603 (m = 4). - Peter Bala, Oct 14 2008 The number of proper mergings of two n-chains. - Henri Mühle, Aug 17 2012 a(n) is number of pairs of non-intersecting lattice paths from (0,0) to (n+1,n+1) using (1,0) and (0,1) as steps. Here, non-intersecting means two paths do not share a vertex except the origin and the destination. For example, a(1) = 3 because we have three such pairs from (0,0) to (2,2): {NNEE,EENN}, {NNEE,ENEN}, {NENE,EENN}. - Ran Pan, Oct 01 2015 Also the number of ordered rooted trees with 2(n+1) nodes and n+1 leaves, i.e., half of the nodes are leaves. These trees are ranked by A358579. The unordered version is A185650. - Gus Wiseman, Nov 27 2022 REFERENCES J. M. Borwein and P. B. Borwein, Pi and the AGM, Wiley, 1987, p. 8. E. R. Hansen, A Table of Series and Products, Prentice-Hall, Englewood Cliffs, NJ, 1975, p. 94. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..100 Abderrahim Arabi, Hacène Belbachir, and Jean-Philippe Dubernard, Enumeration of parallelogram polycubes, arXiv:2105.00971 [cs.DM], 2021. E. Barcucci, A. Frosini and S. Rinaldi, On directed-convex polyominoes in a rectangle, Discr. Math., 298 (2005). 62-78. Paul Barry, On Integer-Sequence-Based Constructions of Generalized Pascal Triangles, J. Integer Sequ., Vol. 9 (2006), Article 06.2.4. Paul Barry, On a Generalization of the Narayana Triangle, J. Int. Seq. 14 (2011), Article 11.4.5. W. Y. C. Chen, S. X. M. Pang, E. X. Y. Qu and R. P Stanley, Pairs of Noncrossing Free Dyck Paths and Noncrossing Partitions, arXiv:0804.2930 [math.CO], 2008. W. Y. C. Chen, S. X. M. Pang, E. X. Y. Qu and R. P Stanley, Pairs of Noncrossing Free Dyck Paths and Noncrossing Partitions, Discrete Math., 309 (2009), 2834-2838. I. Marin and E. Wagner, A cubic defining algebra for the Links-Gould polynomial. arXiv preprint arXiv:1203.5981 [math.GT], 2012. - From N. J. A. Sloane, Sep 21 2012 H. Mühle, Counting Proper Mergings of Chains and Antichains, arXiv:1206.3922 [math.CO], 2012. G. Xin, Determinant formulas relating to tableaux of bounded height, Adv. Appl. Math. 45 (2010) 197-211. FORMULA -4*a(n) = A010370(n+1). G.f.: (1 - E(16*x)/(Pi/2))/(4*x) where E() is the elliptic integral of the second kind. [edited by Olivier Gérard, Feb 16 2011] G.f.: 3F2(1, 1/2, 3/2; 2,2; 16*x)= (1 - 2F1(-1/2, 1/2; 1; 16*x)) / (4*x). - Olivier Gérard, Feb 16 2011 E.g.f.: Sum_{n>=0} a(n)*x^(2*n)/(2*n)! = BesselI(0, 2*x) * BesselI(1, 2*x) / x. - Michael Somos, Jun 22 2005 a(n) = A001700(n)*A000108(n) = (1/2)*A000984(n+1)*A000108(n). - Zerinvary Lajos, Jun 06 2007 For n > 0, a(n) = (n+2)*A000356(n) starting (1, 5, 35, 294, ...). - Gary W. Adamson, Apr 08 2011 a(n) = A001263(2*n+1,n+1) = binomial(2*n+1,n+1)*binomial(2*n+1,n)/(2*n+1) (central members of odd numbered rows of Narayana triangle). G.f.: If G_N(x) = 1 + Sum_{k=1..N} ((2*k)!*(2*k+1)!*x^k)/(k!*(k+1)!)^2, G_N(x) = 1 + 12*x/(G(0) - 12*x); G(k) = 16*x*k^2 + 32*x*k + k^2 + 4*k + 12*x + 4 - 4*x*(2*k+3)*(2*k+5)*(k+2)^2/G(k+1); (continued fraction). - Sergei N. Gladkovskii, Nov 24 2011 D-finite with recurrence (n+1)^2*a(n) - 4*(2*n-1)*(2*n+1)*a(n-1) = 0. - R. J. Mathar, Dec 03 2012 a(n) = A005558(2n). - Mark van Hoeij, Aug 20 2014 a(n) = A000894(n) / (n+1) = A248045(n+1) / A000142(n+1). - Reinhard Zumkeller, Sep 30 2014 From Ilya Gutkovskiy, Feb 01 2017: (Start) E.g.f.: 2F2(1/2,3/2; 2,2; 16*x). a(n) ~ 2^(4*n+1)/(Pi*n^2). (End) a(n) = A005408(n)*(A000108(n))^2. - Ivan N. Ianakiev, Nov 13 2019 a(n) = det(M(n)) where M(n) is the n X n matrix with m(i,j) = binomial(n+j+1,i+1). - Benoit Cloitre, Oct 22 2022 a(n) = Integral_{x=0..16} x^n*W(x) dx, where W(x) = (16*EllipticE(1 - x/16) - x*EllipticK(1 - x/16))/(8*Pi^2*sqrt(x)), n=>0. W(x) diverges at x=0, monotonically decreases for x>0, and vanishes at x=16. EllipticE and EllipticK are elliptic functions. This integral representation as n-th moment of a positive function W(x) on the interval [0, 16] is unique. - Karol A. Penson, Dec 20 2023 EXAMPLE G.f. = 1 + 3*x + 20*x^2 + 175*x^3 + 1764*x^4 + 19404*x^5 + ... From Gus Wiseman, Nov 27 2022: (Start) The a(2) = 20 ordered rooted trees with 6 nodes and 3 leaves: (((o)oo)) (((o)o)o) (((o))oo) (((oo)o)) (((oo))o) ((o)(o)o) (((ooo))) ((o)(oo)) ((o)o(o)) ((o(o)o)) ((o(o))o) (o((o))o) ((o(oo))) ((oo)(o)) (o(o)(o)) ((oo(o))) (o((o)o)) (oo((o))) (o((oo))) (o(o(o))) (End) MAPLE with(combstruct): bin := {B=Union(Z, Prod(B, B))} :seq(1/2*binomial(2*i, i)*(count([B, bin, unlabeled], size=i)), i=1..18) ; # Zerinvary Lajos, Jun 06 2007 MATHEMATICA a[ n_] := If[ n == -1, 0, Binomial[2 n + 1, n]^2 / (2 n + 1)]; (* Michael Somos, May 28 2014 *) a[ n_] := SeriesCoefficient[ (1 - Hypergeometric2F1[ -1/2, 1/2, 1, 16 x]) / (4 x), {x, 0, n}]; (* Michael Somos, May 28 2014 *) a[ n_] := If[ n < 0, 0, (2 n)! SeriesCoefficient[ BesselI[0, 2 x] BesselI[1, 2 x] / x, {x, 0, 2 n}]]; (* Michael Somos, May 28 2014 *) a[ n_] := SeriesCoefficient[ (1 - EllipticE[ 16 x] / (Pi/2)) / (4 x), {x, 0, n}]; (* Michael Somos, Sep 18 2016 *) a[n_] := (2 n + 1) CatalanNumber[n]^2; Array[a, 20, 0] (* Peter Luschny, Mar 03 2020 *) PROG (PARI) {a(n) = binomial(2*n+1, n)^2 / (2*n + 1)}; /* Michael Somos, Jun 22 2005 */ (PARI) a(n) = matdet(matrix(n, n, i, j, binomial(n+j+1, i+1))) \\ Hugo Pfoertner, Oct 22 2022 (Magma) [Factorial(2*n)*Factorial(2*n+1) / (Factorial(n) * Factorial(n+1))^2: n in [0..20]]; // Vincenzo Librandi, Aug 15 2011 (Haskell) a000891 n = a001263 (2 * n - 1) n -- Reinhard Zumkeller, Oct 10 2013 CROSSREFS Cf. A000356, A010370, A038535. Cf. A145600, A145601, A145602, A145603. - Peter Bala, Oct 14 2008 Cf. A000142, A000894, A248045. Equals half of A267981. Counts the trees ranked by A358579. A001263 counts ordered rooted trees by nodes and leaves. A090181 counts ordered rooted trees by nodes and internals. Cf. A000108, A065097, A080936, A185650, A358371, A358586, A358590. Sequence in context: A354018 A154644 A321276 * A242164 A129840 A358214 Adjacent sequences: A000888 A000889 A000890 * A000892 A000893 A000894 KEYWORD nonn AUTHOR N. J. A. Sloane EXTENSIONS More terms from Andrew V. Sutherland, Mar 24 2008 STATUS approved

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Last modified February 21 02:30 EST 2024. Contains 370219 sequences. (Running on oeis4.)