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A051936
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Truncated triangular numbers: a(n) = n*(n+1)/2 - 9.
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10
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1, 6, 12, 19, 27, 36, 46, 57, 69, 82, 96, 111, 127, 144, 162, 181, 201, 222, 244, 267, 291, 316, 342, 369, 397, 426, 456, 487, 519, 552, 586, 621, 657, 694, 732, 771, 811, 852, 894, 937, 981, 1026, 1072, 1119, 1167, 1216, 1266, 1317, 1369, 1422, 1476
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OFFSET
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4,2
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COMMENTS
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Equals binomial transform of [1, 5, 1, 0, 0, 0, ...]. - Gary W. Adamson, Apr 30 2008
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LINKS
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FORMULA
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G.f.: x^4*(-1-3*x+3*x^2) / (x-1)^3.
Sum_{n>=4} 1/a(n) = 53/72 + 2*Pi*tan(sqrt(73)*Pi/2)/sqrt(73). - Amiram Eldar, Dec 13 2022
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EXAMPLE
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Illustration of the initial terms:
.
. . .
. . . o o o
. . o o o o o o o
. o . . o o . . o o o .
. . . . . . o . . . . o o . .
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1 6 12
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MATHEMATICA
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PROG
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(Haskell)
a051936 = (subtract 9) . a000217
a051936_list = scanl (+) 1 [5..]
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CROSSREFS
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KEYWORD
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nonn,nice,easy
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AUTHOR
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Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 21 1999
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STATUS
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approved
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