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A061557
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(7*n+2)*C(n)/(n+2), where C(n) is the n-th Catalan number
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1
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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; internal format)
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OFFSET
| 1,1
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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 (ralf(AT)ark.in-berlin.de), Jan 13 2004
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CROSSREFS
| Cf. A000108, A026009.
A000782 shifted left.
Sequence in context: A193418 A005960 A184120 * A000782 A148775 A148776
Adjacent sequences: A061554 A061555 A061556 * A061558 A061559 A061560
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KEYWORD
| easy,nonn
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AUTHOR
| Darko Marinov (marinov(AT)lcs.mit.edu), May 17 2001
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