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A000782 a(n) = 2*Catalan(n)-Catalan(n-1). 5
1, 3, 8, 23, 70, 222, 726, 2431, 8294, 28730, 100776, 357238, 1277788, 4605980, 16715250, 61020495, 223931910, 825632610, 3056887680, 11360977650, 42368413620, 158498860260, 594636663660, 2236748680998, 8433988655580, 31872759742852, 120699748759856 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(n) = (7n-5)/(n+1) * C(n-1), where C(n) = A000108(n). - Ralf Stephan, Jan 13 2004

Number of Dyck (n+1)-paths that have a leading or trailing hill. - David Scambler, Aug 22 2012

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Guo-Niu Han, Enumeration of Standard Puzzles

Guo-Niu Han, Enumeration of Standard Puzzles [Cached copy]

J. R. Stembridge, Some combinatorial aspects of reduced words in finite Coxeter groups, Trans. Amer. Math. Soc. 349 (1997), no. 4, 1285-1332.

FORMULA

Expansion of (1+x^1*C^1)*C^2, where C = (1-(1-4*x)^(1/2))/(2*x) is g.f. for Catalan numbers, A000108.

Also, apart from initial term, expansion of (1+x^2*C^2)*C, where C = (1-(1-4*x)^(1/2))/(2*x) is g.f. for Catalan numbers, A000108.

a(n) = leftmost column term of M^(n-1)*V, where M = a tridiagonal matrix with 1's in the super and subdiagonals, (1,2,2,2,...) in the main diagonal; and the rest zeros. V = the vector [1,2,0,0,0,...]. - Gary W. Adamson, Jun 16 2011

a(n) = A000108(n+1) - A026012(n-1). - David Scambler, Aug 22 2012

MATHEMATICA

CoefficientList[Series[(1+x*(1-(1-4*x)^(1/2))/(2*x)^1)*((1-(1-4*x)^(1/2))/(2*x))^2, {x, 0, 40}], x] (* Vincenzo Librandi, Jun 10 2012 *)

PROG

(MAGMA) [2*Catalan(n)-Catalan(n-1): n in [1..30]]; // Vincenzo Librandi, Jun 10 2012

CROSSREFS

Partial sums of A071735. Cf. A000108.

Essentially the same as A061557.

Sequence in context: A184120 A215512 A061557 * A148775 A148776 A127385

Adjacent sequences:  A000779 A000780 A000781 * A000783 A000784 A000785

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified December 13 20:27 EST 2019. Contains 329973 sequences. (Running on oeis4.)