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A064322
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Triply triangular numbers.
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5
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0, 1, 21, 231, 1540, 7260, 26796, 82621, 222111, 536130, 1186570, 2445366, 4747821, 8763391, 15487395, 26357430, 43398586, 69401871, 108140571, 164629585, 245433090, 359026206, 516216646, 730632651, 1019283825, 1403201800, 1908167976, 2565535896, 3413156131
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OFFSET
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0,3
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LINKS
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FORMULA
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G.f.: x*(1 + 12*x + 78*x^2 + 133*x^3 + 78*x^4 + 12*x^5 + x^6)/(1-x)^9. [Colin Barker, Apr 19 2012]
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EXAMPLE
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a(4) = 1540 because 4th triangular number is 10, 10th triangular number is 55 and 55th triangular number is 1540.
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MAPLE
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a:= n-> ((k-> binomial(k+1, 2))@@3)(n):
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MATHEMATICA
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f[n_] := n(n + 1)/2; Table[ Nest[f, n, 3], {n, 0, 25}] (* Robert G. Wilson v, Jun 30 2004 *)
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PROG
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(PARI) Tri(n)= { n*(n + 1)/2 } { for (n=0, 400, write("b064322.txt", n, " ", Tri(Tri(Tri(n)))) ) } \\ Harry J. Smith, Sep 11 2009
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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