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A002026 Generalized ballot numbers (first differences of Motzkin numbers).
(Formerly M1416 N0554)
18
0, 1, 2, 5, 12, 30, 76, 196, 512, 1353, 3610, 9713, 26324, 71799, 196938, 542895, 1503312, 4179603, 11662902, 32652735, 91695540, 258215664, 728997192, 2062967382, 5850674704, 16626415975, 47337954326, 135015505407, 385719506620, 1103642686382, 3162376205180, 9073807670316, 26068895429376 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Number of ordered trees with n+1 edges, having root of degree 2 and nonroot nodes of outdegree at most 2.

Sequence without the initial 0 is the convolution of the sequence of Motzkin numbers (A001006) with itself.

Number of horizontal steps at level zero in all Motzkin paths of length n. Example: a(3)=5 because in the four Motzkin paths of length 3, (HHH), (H)UD, UD(H) and UHD, where H=(1,0), U=(1,1), D=(1,-1), we have altogether five horizontal steps H at level zero (shown in parentheses).

Number of peaks at level 1 in all Motzkin paths of length n+1. Example: a(3)=5 because in the nine Motzkin paths of length 4, HHHH, HH(UD), H(UD)H, HUHD, (UD)HH, (UD)(UD), UHDH, UHHD and UUDD (where H=(1,0), U=(1,1), D=(1,-1)), we have five peaks at level 1 (shown between parentheses).

a(n) = number of Motzkin paths of length n+1 that start with an up step. - David Callan, Jul 19 2004

Could be called a Motzkin transform of A130716 because the g.f. is obtained from the g.f. x*A130716(x)= x(1+x+x^2) (offset changed to 1) by the substitution x -> x*A001006(x) of the independent variable. [R. J. Mathar, Nov 08 2008]

REFERENCES

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

T. D. Noe, Table of n, a(n) for n = 0..500

L. Carlitz, Solution of certain recurrences, SIAM J. Appl. Math., 17 (1969), 251-259.

J. B. Cosgrave, The Gauss-Factorial Motzkin connection (Maple worksheet, change suffix to .mw)

R. De Castro, A. L. Ramírez and J. L. Ramírez, Applications in Enumerative Combinatorics of Infinite Weighted Automata and Graphs, arXiv preprint arXiv:1310.2449 [hep-ph], 2013.

R. Donaghey and L. W. Shapiro, Motzkin numbers, J. Combin. Theory, Series A, 23 (1977), 291-301.

Nancy S. S. Gu, Nelson Y. Li, and Toufik Mansour, 2-Binary trees: bijections and related issues, Discr. Math., 308 (2008), 1209-1221.

J. A. Sharp & N. J. A. Sloane, Correspondence, 1977

FORMULA

a(n) = sum(b = 1 to (n+1)/2) [ n choose (2b-1) ][ 2b choose b ]/(b+1).

Number of (s(0), s(1), ..., s(n)) such that s(i) is a nonnegative integer, s(0) = 0 = s(n), s(1) = 1, |s(i) - s(i-1)| <= 1 for i >= 2, Also T(n, n), where T is the array defined in A026105.

a(n) = sum{k=0..n-1, sum{i=0..k, C(k, 2i)*A000108(i+1) }}. - Paul Barry, Jul 18 2003

G.f. 4*z/(1-z+sqrt(1-2*z-3*z^2))^2. - Emeric Deutsch, Dec 27 2003.

a(n)=A005043(n+2)-A005043(n). - Paul Barry, Apr 17 2005

(n+3)*a(n) +(-3*n-4)*a(n-1) +(-n-1)*a(n-2) +3*(n-2)*a(n-3)=0. - R. J. Mathar, Dec 03 2012

a(n) ~ 3^(n+3/2) / (sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Feb 01 2014

MATHEMATICA

CoefficientList[Series[4x/(1-x+Sqrt[1-2x-3x^2])^2, {x, 0, 30}], x] (* Harvey P. Dale, Jul 18 2011 *)

a[n_] := n*Hypergeometric2F1[(1-n)/2, 1-n/2, 3, 4]; Table[a[n], {n, 0, 26}] (* Jean-François Alcover, Aug 13 2012 *)

PROG

(PARI) z='z+O('z^66); concat(0, Vec(4*z/(1-z+sqrt(1-2*z-3*z^2))^2)) \\ Joerg Arndt, Mar 08 2016

CROSSREFS

Cf. A001006, A026300, A026107.

A diagonal of triangle A020474.

See A244884 for a variant.

Sequence in context: A038508 A105695 A244884 * A026938 A086622 A253831

Adjacent sequences:  A002023 A002024 A002025 * A002027 A002028 A002029

KEYWORD

nonn,easy,nice

AUTHOR

N. J. A. Sloane

EXTENSIONS

Additional comments from Emeric Deutsch, Dec 27 2003

STATUS

approved

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Last modified December 12 14:47 EST 2017. Contains 295939 sequences.