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A062741
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3 times pentagonal numbers: 3*n*(3*n-1)/2.
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28
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0, 3, 15, 36, 66, 105, 153, 210, 276, 351, 435, 528, 630, 741, 861, 990, 1128, 1275, 1431, 1596, 1770, 1953, 2145, 2346, 2556, 2775, 3003, 3240, 3486, 3741, 4005, 4278, 4560, 4851, 5151, 5460, 5778, 6105, 6441, 6786, 7140, 7503, 7875, 8256, 8646, 9045
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OFFSET
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0,2
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COMMENTS
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Write 0,1,2,3,4,... in a triangular spiral; then a(n) is the sequence found by reading from 0 in the vertical upward direction.
Number of edges in the join of two complete graphs of order 2n and n, K_2n * K_n - Roberto E. Martinez II, Jan 07 2002
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = log(3) - Pi/(3*sqrt(3)).
Sum_{n>=1} (-1)^(n+1)/a(n) = 2*Pi/(3*sqrt(3)) - 4*log(2)/3. (End)
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EXAMPLE
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The spiral begins:
15
16 14
17 3 13
18 4 2 12
19 5 0 1 11
20 6 7 8 9 10
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MAPLE
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MATHEMATICA
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3*PolygonalNumber[5, Range[0, 50]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 06 2019 *)
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PROG
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(Magma) [Binomial(3*n, 2): n in [0..50]]; // G. C. Greubel, Dec 26 2023
(SageMath) [binomial(3*n, 2) for n in range(51)] # G. C. Greubel, Dec 26 2023
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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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EXTENSIONS
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Better definition and edited by Omar E. Pol, Dec 11 2008
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STATUS
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approved
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