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A135758
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Catalan numbers at triangular positions: a(n) = A000108(n(n+1)/2).
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2
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1, 1, 5, 132, 16796, 9694845, 24466267020, 263747951750360, 11959798385860453492, 2257117854077248073253720, 1759414616608818870992479875972, 5632681584560312734993915705849145100
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OFFSET
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0,3
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LINKS
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Table of n, a(n) for n=0..11.
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FORMULA
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a(n) = C(n(n+1), n(n+1)/2) / [n(n+1)/2 + 1].
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MATHEMATICA
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With[{nn=100}, Join[{1}, Pick[CatalanNumber[Range[nn]], Array[ IntegerQ[ (Sqrt[1+8#]-1)/2]&, nn]]]] (* From Harvey P. Dale, Sep 24 2011 *)
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PROG
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(PARI) a(n)=binomial(n*(n+1), n*(n+1)/2)/(n*(n+1)/2+1)
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CROSSREFS
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Cf. A135757.
Sequence in context: A191409 A003372 A187357 * A152794 A053517 A114474
Adjacent sequences: A135755 A135756 A135757 * A135759 A135760 A135761
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KEYWORD
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nonn
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AUTHOR
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Paul D. Hanna, Dec 02 2007
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STATUS
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approved
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