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A066370
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Quadruply triangular numbers.
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3
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0, 1, 231, 26796, 1186570, 26357430, 359026206, 3413156131, 24666759216, 143717956515, 703974775735, 2989908659661, 11270904497931, 38398515291136, 119929709686710, 347357071281165, 941718655098991, 2408309883851256
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| See sequences A000217, A002817 and A064322 for triangular, doubly triangular and triply triangular numbers, respectively.
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FORMULA
| a(n) = n*(n + 1)*(n^2 + n + 2)*(n^4 + 2*n^3 + 3*n^2 + 2*n + 8)*(n^8 + 4*n^7 + 10*n^6 + 16*n^5 + 25*n^4 + 28*n^3 + 28*n^2 + 16*n + 128)/32768.
a(n) = A064322(n)*(A064322(n) + 1)/2 = A000217(A000217(A000217(A000217(n))))
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EXAMPLE
| a(2)=231 as A000217(A000217(A000217(A000217(2))))=231 is the second quadruply-triangular number.
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MATHEMATICA
| f[n_] := n(n + 1)/2; Table[ Nest[f, n, 4], {n, 0, 17}] (from Robert G. Wilson v Jun 30 2004)
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CROSSREFS
| Sequence in context: A183663 A201957 A152500 * A139412 A027551 A172861
Adjacent sequences: A066367 A066368 A066369 * A066371 A066372 A066373
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KEYWORD
| nonn
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AUTHOR
| Brian Bayerle (bbayer03(AT)providence.edu), Dec 22 2001
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