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A094159
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3 times hexagonal numbers: a(n) = 3*n*(2*n-1).
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25
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0, 3, 18, 45, 84, 135, 198, 273, 360, 459, 570, 693, 828, 975, 1134, 1305, 1488, 1683, 1890, 2109, 2340, 2583, 2838, 3105, 3384, 3675, 3978, 4293, 4620, 4959, 5310, 5673, 6048, 6435, 6834, 7245, 7668, 8103, 8550, 9009, 9480, 9963, 10458, 10965, 11484
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OFFSET
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0,2
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COMMENTS
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Sequence found by reading the line from 0, in the direction 0, 3, ..., in the square spiral whose vertices are the generalized pentagonal numbers A001318. - Omar E. Pol, Sep 08 2011
a(n) is the sum of all perimeters of triangles having two sides of length n. For n=4 one has seven triangles with two sides of length 4 and the other of lengths 1..7. - J. M. Bergot, Mar 26 2014
a(n) is the Wiener index of the complete tripartite graph K_{n,n,n}. - Eric W. Weisstein, Sep 07 2017
Sequence found by reading the line from 0, in the direction 0, 3, ..., in a spiral on an equilateral triangular lattice. - Hans G. Oberlack, Dec 08 2018
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REFERENCES
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Dan Hoey, Bill Gosper and Richard C. Schroeppel, Discussions in Math-Fun Mailing list, circa Jul 13 1999.
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LINKS
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FORMULA
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Sum_{n>=1} (-1)^(n+1)/a(n) = Pi/6 - log(2)/3. - Amiram Eldar, Jan 10 2022
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MAPLE
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MATHEMATICA
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CoefficientList[Series[3x(1+3x)/(1-x)^3, {x, 0, 50}], x] (* Vincenzo Librandi, Jun 19 2013 *)
Table[3n(2n-1), {n, 0, 50}] (* or *) 3*PolygonalNumber[6, Range[0, 50]] (* or *) LinearRecurrence[{3, -3, 1}, {3, 18, 45}, {0, 50}] (* Eric W. Weisstein, Sep 07 2017 *)
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PROG
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(Magma) [3*n*(2*n-1): n in [0..50]]; // G. C. Greubel, Dec 07 2018
(Sage) [3*n*(2*n-1) for n in range(50)] # G. C. Greubel, Dec 07 2018
(GAP) List([0..50], n -> 3*n*(2*n-1)); # G. C. Greubel, Dec 07 2018
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CROSSREFS
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Cf. numbers of the form n*(n*k-k+6))/2, this sequence is the case k=12: see Comments lines of A226492.
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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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Definition improved, offset corrected and edited by Omar E. Pol, Dec 11 2008
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STATUS
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approved
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