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A132081 Triangle (read by rows) with row sums = Motzkin sums (also called Riordan numbers) (A005043): T(n,s) = (1/n)*C(n,s)*(C(n-s,s+1) - C(n-s-2,s-1)). 7
1, 1, 2, 1, 5, 1, 9, 5, 1, 14, 21, 1, 20, 56, 14, 1, 27, 120, 84, 1, 35, 225, 300, 42, 1, 44, 385, 825, 330, 1, 54, 616, 1925, 1485, 132, 1, 65, 936, 4004, 5005, 1287, 1, 77, 1365, 7644, 14014, 7007, 429 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,3

COMMENTS

Whereas A005043 counts certain trees, or noncrossed partitions, this subdivides the counts according to the number of leaves, or the lattice rank. Analogous to the Narayana triangle (A001263), where rows sum to the Catalan numbers.

Diagonals of A132081 are rows of A033282. - Tom Copeland, May 08 2012

Related to the number of certain non-crossing partitions for the root system A_n. Cf. p. 12, Athanasiadis and Savvidou. See also A108263 and A100754. - Tom Copeland, Oct 19 2014

LINKS

Table of n, a(n) for n=3..50.

C. Athanasiadis and C. Savvidou, The local h-vector of the cluster subdivision of a simplex, arXiv preprint arXiv:1204.0362 [math.CO], 2012.

F. R. Bernhart, Catalan, Motzkin and Riordan numbers, Discr. Math., 204 (1999), 73-112.

F. R. Bernhart & N. J. A. Sloane, Emails, April-May 1994

T. Copeland, Generators, Inversion, and Matrix, Binomial, and Integral Transforms, 2015.

FORMULA

a(n,k) = binomial(n,k)*binomial(n-2-k,k)/(k+1). - David Callan, Jul 22 2008

From Peter Bala, Oct 22 2008: (Start)

O.g.f. : 1 + x + sqrt(1 - 2*x + x^2*(1 - 4*a))]/(2*x*(1 + a*x)) = 1 + a*x^2 + a*x^3 + (a + 2*a^2)*x^4 + (a + 5*a^2)*x^5 + (a + 9*a^2 + 5*a^3)*x^6 + ... .

Define a functional I on formal power series of the form f(x) = 1 + a*x + b*x^2 + ... by the following iterative process. Define inductively f^(1)(x) = f(x) and f^(n+1)(x) = f(x*f^(n)(x)) for n >= 1. Then set I(f(x)) = lim n -> infinity f^(n)(x) in the x-adic topology on the ring of formal power series; the operator I may also be defined by I(f(x)) := 1/x*series reversion of x/f(x).

Let now f(x) = 1 + a*x^2 + a*x^3 + a*x^4 + ... . Then the o.g.f. for this table is I(f(x)) = 1 + a*x^2 + a*x^3 + (a + 2*a^2)*x^4 + (a + 5*a^2)*x^5 + (a + 9*a^2 + 5*a^3)*x^6 + ... . Cf. A001263 and A108767. (End)

EXAMPLE

A005043(6) = 15 = 1+9+5 since NC (noncrossed, planar) partitions of 6-point cycle without singletons have 1,9,5 items with 1,2,3 blocks.

Triangle begins:

  1;

  1,   2;

  1,   5;

  1,   9,   5;

  1,  14,  21;

  1,  20,  56,  14;

  1,  27, 120,  84;

  1,  35, 225, 300,  42;

  1,  44, 385, 825, 330;

  ...

MATHEMATICA

Map[Most, Table[(1/n) Binomial[n, s] (Binomial[n - s, s + 1] - Binomial[n - s - 2, s - 1]), {n, 3, 14}, {s, 0, n}] /. k_ /; k <= 0 -> Nothing] // Flatten (* Michael De Vlieger, Jan 09 2016 *)

PROG

(MAGMA) /* triangle excluding 0 */ [[Binomial(n, k)*Binomial(n-2-k, k)/(k+1): k in [0..n-3]]: n in [3..15]]; // Vincenzo Librandi, Oct 19 2014

CROSSREFS

Row sums are A007404.

Cf. A001263, A005043, A033282, A100754, A108263, A108767, A132081.

Sequence in context: A178470 A093127 A115123 * A054251 A163963 A119763

Adjacent sequences:  A132078 A132079 A132080 * A132082 A132083 A132084

KEYWORD

nonn,tabf

AUTHOR

Frank R. Bernhart (farb45(AT)gmail.com), Oct 30 2007

EXTENSIONS

Edited by N. J. A. Sloane, Jul 01 2008 at the suggestion of R. J. Mathar

Name corrected by Emeric Deutsch, Dec 20 2014

STATUS

approved

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Last modified November 15 15:59 EST 2018. Contains 317239 sequences. (Running on oeis4.)