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A061557 (7*n+2)*C(n)/(n+2), where C(n) is the n-th Catalan number 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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The identity a(n)=Sum(3*(k-1)*C(k)*C(n-k)/(2*k-1),k=0..n) was verified using Wilf-Zeilberger theory for hypergeometric sums. The sum arises in the enumeration of separable 1324-avoiding permutations: A026009(n)=a(n)/2+2*C(n-1)-5*C(n)/2.

a(n) = 2C(n+1) - C(n), with C(n) = A000108(n). - Ralf Stephan, Jan 13 2004

LINKS

Table of n, a(n) for n=1..24.

CROSSREFS

Cf. A000108, A026009.

A000782 shifted left.

Sequence in context: A273716 A184120 A215512 * A000782 A148775 A148776

Adjacent sequences:  A061554 A061555 A061556 * A061558 A061559 A061560

KEYWORD

easy,nonn

AUTHOR

Darko Marinov (marinov(AT)lcs.mit.edu), May 17 2001

STATUS

approved

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Last modified February 19 18:30 EST 2019. Contains 320327 sequences. (Running on oeis4.)